The Offshore Voyaging Reference Site

Cruising Boat Electrical System Design, Part 1—Loads and Conservation

Two core decisions we must make when designing a cruising boat’s electrical system for living aboard full-time and making offshore voyages are:

  • the size of the battery bank, and
  • which charging sources we will need: generator, solar, main engine, wind, etc.

But the first thing we need to do, before getting into all that fun stuff, is think about electrical consumption and how to keep it reasonable.

That’s what I’m going to cover in this chapter, and then in the next we will move on to battery bank specification.

It’s Worth It

Whatever your situation, do make the effort to go through this process, particularly if you have not yet got out there cruising and/or done a multi-day passage, since unrealistic assumptions about power usage, leading to undersize battery banks and charging sources, is one of the most common cruise and/or voyage ruiners.

And even for me, after all my years of cruising successfully on the same boat, this exercise has proved both interesting and useful.

The Old Way

Typically, when designing an electrical system, we are supposed to fill out a form listing every single load on the boat in order to calculate our expected consumption.

But there are several problems with that approach:

  • It pushes us into thinking about the details ahead of the big picture—pretty much always a bad idea.
  • It’s difficult to estimate electrical usage for a given piece of kit. Sure we can look up the specification, but that doesn’t help much unless we can also make an accurate assessment of how much of the time the machine in question will be running, and how its consumption may vary over time; not always, or even often, easy.
  • On a modern cruising boat, with scores of machines that use electricity, this kind of bottom-up analysis will take a huge amount of time.

A Better Way

Given that, I’m going to come at this from the top down, by looking at the loads that really matter, along with rules of thumb for estimating them based on my some-25 years of living on boats.

Will this approach be dead-nuts accurate? No, but it will let us get as close as we need to. And, anyway, the inaccuracies in the bottom-up approach are probably as bad or worse—just because we write down a lot of numbers doesn’t make ’em right.

Do We Even Need To Fix It?

The other advantage of this approach is that we can reverse the process by using the same rules of thumb to understand what level of electrical consumption the system we already have will support (more on how in Part 2). Increasing battery bank size and charging sources is a huge project, at least when done right, that can seriously screw with our cruising budget both money- and time-wise.


When thinking about at-sea loads, I will be primarily focusing on sailboats, since a motorboat, at least one with proper alternators, does not need to worry about loads while underway. But I will also cover loads at anchor, which will be of interest to both sail and power owners.

  • I will be expressing usage, load, and capacity in amp hours (Ah) for a 12-volt system. If you have a 24-volt system just divide my numbers by two.
  • When I write “day” I’m referring to 24 hours, unless otherwise stated.

Power Suckers

Let’s start by looking at the four horsemen of the flat battery: 

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More Articles From Online Book: Electrical Systems For Cruising Boats:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Fusing
  34. Lithium Buyer’s Guide—Budget: High End System
  35. Lithium Buyer’s Guide—Budget: Economy Options
  36. 10 Reasons Why Hybrid Lithium Lead-Acid Systems are a Bad Idea
  37. 11 Steps To Better Lead Acid Battery Life
  38. How Hard Can We Charge Our Lead-Acid Batteries?
  39. How Lead Acid Batteries Get Wrecked and What To Do About It
  40. Equalizing Batteries, The Reality
  41. Renewable Power
  42. Wind Generators
  43. Solar Power
  44. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  45. Battery Monitors, Part 1—Which Type Is Right For You?
  46. Battery Monitors, Part 2—Recommended Unit
  47. Battery Monitors, Part 3—Calibration and Use
  48. Battery Containment—Part 1
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Matt Marsh

John, I’m glad you’ve emphasized the importance of being realistic and honest in estimating electrical consumption.

It’s really easy to add everything up, get something like 6 kWh/day ( = 250 Ah/day @ 24 V, 500 Ah/day @ 12 V), and think “My gosh, that’s like a thousand pounds of batteries, that can’t be right!” So you go back and say “well, the autopilot won’t really take that much, and the radar won’t be on that long” until you get the number you want.

But, if you’re honest about your consuption up front, you end up with a much better balanced system than if you try to cheat the input data. You decided up front that you wanted to run the radar all night on passage, and that you wanted to run under autopilot 24/7 offshore, and that you wanted to go 3-4 days at anchor before smelling diesel fumes. Since those desires formed the basis of the design, your system can handle them without a fuss. It’s a much more relaxing position to be in than watching the breeze come up and wondering “do I really have enough juice in those two 8Ds for this autopilot to last the night?”

Jim Evans

Toasters? Pah! European boats often have a grill under the stove burners: great for toast, a chop, fish – the list goes on. I don’t know why we never see them on North American boats, but they’re great. Failing that, a large coffee can with a wire grid over the top and holes punched in the bottom, set over a stove burner, makes great toast fast. It’s not elegant, but it’s efficient.
And as for microwaves, they’re only any use if you have a freezer or take a trip to the supermarket every day: it’s wonderful how removing one system can remove the need for another.
Mind you, having lived for years without, I have a secret hankering for refrigeration…
A great article (again) though, John. Your remarks about autopilots are particularly germane: it’s all too easy to get into a situation where autopilot use drains the batteries and you get exhausted steering. Frequent attention to sail trim really pays off.

Drew Frye

Ironically, I use more power in the winter, when I have less solar. I run the lights more, I tend to watch movies, and the heater draws power. The propane solenoid is also a sneaky one.

Matt Boney

Thanks for another excellent article John, but I have to comment on methods to arrive at an average daily consumption. There are long spreadsheets online to help calculate the Ah usage of every piece of electrical equipment, but any calculation on the current consumption and the time used, as you say, will be mainly guesswork. In my opinion the only accurate way is to measure the Ah discharged using a shunt based Battery Monitor. All cruising boats should have one of these but it must be correctly installed to measure the current taken by all equipment on the boat.

So by turning off all loads and all charging sources there should be zero current draw, but add any piece of equipment like radar or the fridge and the current draw will be shown immediately. Turning off wind and solar should be easy but disabling the engine alternator for 24 hours is the hard part. This can be done by removing the 12v supply that powers any regulator via the ignition switch. On some engines the regulator can be disabled by simply turning off the ignition switch, but this will disable any electrical instruments or fuel pump used by the engine. Some regulators once exited can’t be disabled this way.

Depending on how you use your boat accurate measurements can now be made of the real Ah used by all equipment with all charging sources disabled to determine the worst case scenario.

Sailing over 24 hrs
Anchoring 24 hrs
Anchoring overnight
Sailing overnight only
Sailing daytime only

These need to be done several times to obtain an average. I have to admit that I have only done anchoring overnight – but this depends on how much the computer is used, especially watching movies!

Oliver Schonrock

This is beyond the scope of this excellent article (looking forward to Part II), and sorry for the long comment. Perhaps there is another article that covers it, which I haven’t found, or perhaps a prompt to share your experience…

Anyway. Fridges/Freezes and their power consumption. I have spent about 10 years fighting this problem on a range of cruising boats. Because I have a degree in Electrical Engineering, that one always landed on my desk, whichever boat I was on. Oliver, we can’t seem to keep the fridge/freezer cold and/or the batteries charged. Can you fix it? Sigh. The many other electrical/electronic problems on boats, which I was asked to help with were usually pleasantly easy to solve in comparison.

I know you don’t like engine driven compressors and I understand why and mostly agree. However, there is a really key feature, which, if we can replicate it without the belt driven monster, can revolutionize a cooling system.

That feature is: Very high cooling output power “for free”. Dumped into holding plates, as you suggest above. My experiences are quite dated (over 20yrs old). But not THAT much has changed I think. This essentially allows us to use the holding plates as a “better kind of battery” just for that purpose. A “battery” which can accept charge at a very high rate, until it’s “full”. Big compressor, no problem => max power right in there. Power to drive that big compressor is “free” when the engine is running (or the genset, see below). So the question is how can we achieve these benefits without the engine driven compressor nightmare.

I haven’t quite been able to glean exactly what your setup is on MC. But what I understand, is that you have holding plates in your fridge and freezer and a single big 12VDC compressor (judging by the size of it on your photos, maybe 1/2 hp or even 3/4 Hp??, ie 375 – 600W or 30-50A at 12V). So that’s good, and has some of the features. When your genset is running you can cool down those plates, rapidly and “it’s for free” because the genset has spare capacity and is running anyway. Extra benefit: it loads the genset nicely. Ideally the compressor would be even bigger to take even more power when it’s “freely” available. Ideally it would also have an automatic “always run when the genset is on” wire to make sure you take full advantage (needs thermal protection for the plates and compressor!).

First shortcoming: Because your compressor is 12VDC, and your genset is 120VAC you have to install extra “battery charger” capacity to take full advantage of this. The 3x 40A battery chargers I remember reading about elsewhere, don’t quite seem to do that. Are you not compromising your max bulk battery charging rate for the batteries because the fridge/freezer compressor is running? Need more battery chargers? (ie make more amps available at 12VDC when the genset is running). The same applies to the engine alternator. We to make it big, really big. Or have 2. – Again, the belt driven refrigeration compressor monster is suddenly appealing, but we know why not.

The second problem: Lead acid batteries suck. Badly. As you have eloquently written about elsewhere. Basically any power that you need to put there in order to take it out again costs you. Big time, for a range of reasons, which I won’t repeat here. So when your genset is not running, or, as on Colin’s Ovni, there is no genset but more low power alternative energy sources (MC has some of those too, but less). The best way to reduce the lead-acid nightmare is to not put it in there in the first place. ie try to consume the power when it’s available. In terms of refrigeration compressors this means that a 30-50A fridge compressor is FAR from optimal, because you are very unlikely to produce that much “spare” from low power alternative energy source. So what you want here is a 120W (10A@12VDC) compressor at most, less on a smaller boat with smaller fridge/freezer.

So the conundrum of avoiding the black hole which is the lead-acid bank, is to be able to cool the fridge/freezer in 2 modes: VERY High power when the engine or genset is running, and only low to moderate power when it’s not – using up the amps available from solar, wind and hydrogens.

Do agree with the above analysis? Do you know a good solution to this problem?

I know that these days there are “multivoltage” compressors available (ie 120/240VAC or 12VC). I am not sure that solves it, because in 12VDC mode you would still be sucking far too much power. The only idea I have is: Have 2 compressors. one high power connected to the plates. And one low power on an evaporator. The low power one will be 12VDC. The high power one could be 120/240VAC if you have a genset (although then it doesn’t work on the engine – except through an inverter). Or the high power one could be 12VDC (like yours) but then you need extra battery chargers for the genset.

We can also think of the problem in “Joules or Watt hours of Energy” (Watts multiplied by time running – similar as Ah, but not dependent on voltage). The 600W compressor running for 1hour while the genset is on, could be complimented by the 100W 12VDC low power compressor running for 6 hours while solar/wind/hydro power is available. In each case you have 600Whr of electrical energy available for refrigeration. With NO lead-acid black hole involved.

Other ideas? Look forward to your input.

PS: Have you ever seen a 120/240VAC generator driven by the main engine? Is that more realistic than 2 160A alternators?


Hi Oliver,

In my experience your analysis regarding the potential advantages of dual compressor modes is correct. Our boat came equipped with a SeaFrost system that cooled both a fridge and a separate freezer with dual-circuit holdover plates. By dual-circuit I mean there are two separate cooling coils inside each holdover plate that are not connected to each other in any way – four copper tube ends exit each plate. On each holdover plate one of the two circuits was cooled by a high-capacity engine driven compressor, similar to an automotive AC compressor, and the second of the two circuits was cooled by a moderately power hungry 120 Volt AC compressor. We essentially never used the 120 Volt AC system as we do not have a generator and our boat is rarely tied to a dock. We could have powered the AC system with the inverter but the Amp draw would have been in a punishing 30 to 40 Amps range. As we began taking longer cruises it didn’t take long for us to arrive at your analysis: we wanted to be able to rapidly transfer surplus energy from the engine into the refrigeration system when the engine was running. And when the engine was not running we wanted a 12 Volt DC system that would operate at an efficient low Amp draw such that it would be completely powered by our 300 Watt solar system during sunlight hours and not incur the inefficiencies involved in charging and discharging lead-acid batteries during that time.

After evaluating several different paths and consulting with Cleave and Nate from SeaFrost we elected to retain the engine driven compressor system and to replace the 120 Volt AC compressor system with two completely separate 12 Volt DC compressors, one for the fridge and one for the freezer.

We are pleased with the efficiency and versatility of the upgraded system. Less obvious, there are a series of redundancies inherent to the upgraded system. We can shut down the freezer and run just the fridge, saving considerable energy. We can utilize the freezer as a back-up fridge and shut down the fridge should a component of the fridge system fail. We can operate on 12 Volts DC alone should the engine or a component on the engine driven side fail. We can operate on the engine driven system alone should we have a partial or complete electrical side failure. The DC compressors are efficient enough that they do not add significant load to the battery charger during the infrequent times when we are plugged into shore power, so they provide everything the AC compressor offered plus much more. Lastly, the DC compressors will cool to either sea water or air, a redundancy we appreciated when an unexpected maintenance haul-out was required and we lived on the hard for a while.

The real impact is that we don’t think much about the refrigeration system any more. If we’re motoring we try to remember to run the engine driven system for the last 45 minutes before we shut the engine off. Then, after half a day or so, depending on the climate, when the holdover plates have mostly thawed, the DC system takes over and keeps the box(s) cool going forward. In the Bahamas, with our dark hull, 300 Watts of solar was enough to cool the fridge alone indefinitely without the engine. If we ran both the fridge and the freezer we would slowly loose ground and our 450 Amp/hour house bank would need charging every two days or so. Our next upgrade will be some additional solar capacity but we still like having that engine driven compressor under the sole.

Oliver Schonrock

Hi Grinnel

That does sound like a nice setup.

I like the redundancy, and the multiple choices for tapping into power to get things chilled at the rate at which the power is currently available. I have seen dual circuit holding plates before, just not had the privilege of cruising on a boat which had them. And yes those engine mounted compressors are basically automotive AC units (and can be quite reasonably priced).

John has good points too. Engine mounted anything reduces access, increases complexity and puts vibration where you don’t want. Even the obligatory alternator, is slightly problematic in this respect. John is also right, that his system is certainly simpler with less components, piping, joints etc than yours.

But then again, I have seen plenty of boats which make the engine mounted thing work and work reliably, and you have found that too. Those Automotive AC units do bounce around under bonnets of cars for their whole life normally, and have similar issues re flexible connections under vibration etc.

12VDC enclosed compressors have become somewhat more efficient over the last 20 years, so perhaps the engine mounted monster is less required? But great if you have it because it gives you options, and as you said “cooling ceases to be an issue at all”, without any lead-acid impact to boot – which we can then use to run those myriad of computers and other similar low to medium power devices that present a more suitable load for lead-acid.

I would always try to get a windvane to work with any boat that I cruise in, and have good experience with those up to 60ft. So on passage, it should be possible to achieve a power surplus due to the hydrogen options available. That surplus can be used for the low power 12VDC refrigeration system. No engine or genset required, ideally for the whole passage.

At Anchor high power topup would be required (as you have found too). On a genset boat that would be maybe a 1-1.5HP 120/240VAC compressor for me – I spent my youth maintaining high power low voltage DC motors and I dislike them. Without a genset, need some high power way to tap into engine power. Lots of 12VDC alternator juice or 120/240VAC engine driven generator or the engine mounted compressor. Running the engine for an hour every 1-2 days seems reasonable to me. And often naturally happens by motoring on/off anchor from one bay to the next.

Your point about 12VDC working nicely on shore power in the boatyard if you ensure you have an air cooled condenser as one of your options, it a really good one. I didn’t know you could plumb in something which cooled the condenser with “either water or air”. How does that work? Is that with a “double 3 way valve” of some sort?

I know there is lots of content on this site about high latitude cruising. That’s great, but obviously refrigeration is much easier in colder climes. I think my next trip will be in the heat, lots of it, not only because my cruising partner likes it that way. So powerful refrigeration is essential. If you’re going to have refrigeration at all, then you need to make it work properly and not just have a luke warm coolbox.

Thanks for sharing you experiences. It’s a complex subject.



Hi Oliver,

This is a response to your response. The site’s comments engine does not appear to allow comments indented more than 4 layers deep, so I reply here and hope you see it.

To answer your questions:
The two 12 Volt DC compressors are SeaFrost Tradewinds XP Air-Water Condensing Units. They feature both a small refrigerant to air heat exchanger (looks just like a small heater core) as well as a compact refrigerant to water heat exchanger. The boat was already equipped with the through hulls and strainer for the old water cooled 120 Volt AC condenser unit. It was therefore an easy decision to specify the water cooling option on the 12 Volt Tradewinds units and utilize the legacy through-hulls to achieve greater cooling efficiency.

The engine driven compressor is not mounted directly to the engine but rather to a bracket adjacent to the engine. Therefore, it is not subject to engine vibrations. However, it is driven by a rather long belt from a pulley on the engine and that belt is subject to considerable vibration.

Philip Wilkie


With a similar background to yourself I’m equally unimpressed with Lead Acids myself and I’m committed to find better solutions myself.

In terms of scheduling refrigeration properly I completely agree with you. This Australian company seems to have thought all this through very nicely:



Hi John
Of the four evil energy users onboard cruising boats the electric autopilot stands at the head of the list.

Except that by observation many (most?) cruising sailboats rarely go anywhere, and if they do they usually motor instead of bothering to put up the sails. So that $5,000 autopilot connected directly to the quadrant is the ideal tool for the job and its power thirst is unimportant.

For the subset of cruisers who are actually sailors, the sophistication of electronics and controls of the modern autopilot make it a very convenient device— one that is reliable most of the time until Murphy pays you a visit in the middle of the night far from land. It is not rocket science to design an electric autopilot that requires magnitudes less electrical power buy using the energy of the vessel moving through the water to control a trim tab or servo paddle. But neither wind vane manufacturers or electronic autopilot manufacturers specifically design for this use. Is there no demand or are we caught in the common human trait of conventional thinking?

Oliver Schonrock

I think you will find that most windvane suppliers do support solutions of using low power tiller pilots with the servo pendulum generating the power. This should not be the primary mode of operation, but works as a backup or in very light wind.

You are right that there are many “motorsailing cruisers” out there. For these people, autopilots and also refrigeration are much less of an issue.

For those of us that like the sailing bit – and I include myself here – electrical power is always a problem that requires careful design and planning, hence Johns article here. For the keen sailors who understand how to trim their boat and enjoy that, it is my view that a windvane should be one of the self-steering systems on board, and if well chosen and installed it should be the primary one – meaning it steers more than 85% of the time on passage.

Oliver Schonrock

Hi John

4 level indent limitation here. Replying to your comment above about dangers/problems with engine compressor.

I completely agree, as I hope I made clear, that attaching anything to front of main engine is a headache.

People, even the suppliers of such kit, seem to talk about mounting compressor on stringers in boat, not on engine frame. That seems mad to me. Those reference points move by 2 inches relative to each other sometimes (at certain low rpm resonance frequencies). I would be really not keen to do that, although Grinnel seems to have had a positive experience.

Can we instead focus on what the problems are – and am still conscious of article scope creep here – with mounting an engine driven refrigeration compressor or indeed a 120/240VAC generator (actually those don’t really exists, it’s a 240VDC alternator with an inverter to make the right (50/60Hz) frequency) suspended on a frame which is attached to the same reference frame as the engine crankshaft, above the flexible mounts.

This is no different to mounting a high output 200A alternator to the engine, is it? I know it needs flexible pressurized refrigerant hoses. But other than that? What is the concern? (I personally have some, but I am trying to not pre-bias the discussion).


Hi John,
Yes, there have been many adaptions of cheap tiller pilots to operate wind vanes. The result usually has been a light weight system that is somewhat fiddly to use. Nothing like pushing a button in the cockpit and returning to your rum punch.

The point I made was (to my knowledge) there are no systems designed as primary autopilots that make use of hydraulic forces to radically reduce electrical load.

Back in the day an Italian firm called Mustafa offered a massively strong auxiliary rudder vane that weighed 60 kilos. Kind of a Hydrovane on steroids. It has long since become history, but that is one kind of kit that could be the basis for a stand alone hydraulic servo assisted electric autopilot.

Oliver Schonrock

I am not aware of that system. How did it work exactly? Do you know why it did not survive?

Rob Gill

Hi John,
Some savings that helped us not replace our generator when it retired:
Replaced a solenoid that ran hot keeping our gas valve open when the stove was in use, with a new BEP unit that stays cool by only sending a signal – saving about 2 amps.
Shutting down the radar fully (default is standby) when not needed – saving 1.5 amps.
At anchor, stainless solar path lamps that come on a stainless stalk ($6 from hardware store) stowed in fishing rod holders on the bow and each quarter lighting the boat and the cockpit (the bow rod holder also serves to hold the washdown hose). Helpful too when I forget our masthead anchor light and for identifying Bonnie Lass in a busy anchorage at night. Saving ~ 1->2 amps. Beware, they can sometimes turn-off around 3-> 4 a.m on a cloudy day.
Turning off the inverter when not in use ~ 1.25 amps (current used by the unit to stay alive).
Nav station chart plotter to sleep mode when not in use ~ 1.5 amps, even though it is the master unit and contains the charting SW micro card.
De-tune the autopilot upwind and reaching to stop it oversteering – saving about 2->3 amps.
Increase autopilot sensitivity downwind and broad-reaching to encourage more frequent, smaller helm movements and avoid large corrections being needed from the hydraulic pump – saving around 2->4 amps.
Changing from old water cooled freezer compressor to new air cooled unit plus better insulating the box and re-sealing the lid – saving about 6 amps.
Since some of these power drains impact over 12 or 24 hours they do add up. Anyone got their successful power saving ideas or tips to share on the load side of the equation?


Would you please have a look at
They have just won 2018 DAME OVERALL AWARD WINNERS
It looks good but I wonder about the loads on an old engine which I presume would be involved.
And maybe putting too many eggs in one basket.

Oliver Schonrock

I agree with John:

– A belt-based PTO for 5-9kW is pretty scary
– This is a very custom and complex system and the 48V base-bus voltage is a kludge.

But there are potentially ideas here that have merit – particular the “take power where we can approach” at low end to load engine without the prop, or at high end because we almost never run our engines flat out. My line of thinking is:

– Even 48V is too low for a (up to) 9kW generator. That’s 180A flowing in that alternator (@ 100% efficiency, so it will be more). Hence the heat problems. That sort of power is better generated at 240V or even 400V. (like in a genset) which would reduce current and hence wire-size/heat.

– There is a somewhat similar product which does this at 230V. The machine makes variable frequency AC (because the engine speed varies). They rectify that to make ~240V DC (average loaded root mean square – never mind the detail here, but you can trust me that’s right). They then have a box of power-electronics to chop up the DC and make nice’ish fixed 50Hz 230VAV (using European numbers here). This is like an inverter except no voltage boost circuitry is required because we start with a high voltage.

– So that’s great, and we could run our kettle and hair dryer and 230V fridge compressor, off this. But what about batteries? Well we would need to re-rectify the 50Hz 230VAC back into DC while reducing the voltage (done with ~5% duty cycle PWM at abuot 20kHz frequency – never mind the detail) to gives us 12-14.4VDC for our batteries – Yes, it’s a “shore-power battery charger”.

– If we are prepared to go with a slightly more “custom box of electronics” (and John’s point is well made – the more standard the better, but lets continue). We could skip 2 steps for the 14VDC battery charger output. ie from our 240VDC (loaded RMS) “Bus” volatage, we would have 2 “take off’s”…One to make 230VAC for those applianced which are “high-power” eg large fridge compressors, or hair dryers, or diving compressors etc.. And a second one to make 14VDC for battery charging (skipping the step of making AC only to re-rectifying it into low voltage DC).

At a high level I think this would be a superior architecture to that 48V system, because:

– We get the 2 voltages which we want (because they have commonly available applianced) 12VDC and 240VAC.

– The currents are only high where they need to be (on the 14VDC battery charging output)

– We are not making another low-voltage bus (48VDC), which we don’t actually want.

Is this rocket science or black magic? No, not at all. Power electronics are a very mature technology (we use them already in Inverters and Battery chargers). Can they be configured to operate as above. Yes, they absolutely can. (My Masters thesis is on this subject).

Are their lots and lots of control system, safety and practical problems to solve?


But does this sound worthwhile to explore?

Oliver Schonrock

I agree that 5kW is lots, unless you have something seriously power hungry like a diving compressor. I also think the “up to 9kW” proposition from Integrel is
a) just what is says in the video, not real / average output and
b) unnecessary.

I have also spent quite a bit of time on cruising boats with and without generators (not nearly as much as you!). I look at at this way:

1. I am working on the assumption that what we all hate is the rumbling of a diesel, genset or main (even though gensets are typically much more quiet, and that’s a big problem for anything main engine driven, including Integrel or Elja). And we hate maintenance, produced by complex systems or numerous systems.

2. The problem with any diesel is that when it’s running you typically (see below) have more power than you can use so it remains underutilised and you end up running it for longer per day than you would like. Very frustrating.

3. There are, in principle only two solutions to item 2 above (apart from just using less power, or using alternative energy sources).
a) get a bigger lead-acid bank (and lots of battery chargers), which is expensive on a repeat basis and brings with it many problems of sulphating etc which you have descibed so well elsewhere.
b) use more power when the diesel is running so you need less lead-acid.

4. My proposal from above is that a high power (possible in the order of 1.5kW) refrigeration unit – one of the two big power hogs – with big holding plates, is a good target for utilising more diesel power => being able to run it less per day and requiring only a smaller lead-acid bank.

5. Here I am arguing purely from my Electrical training. It’s not about AC or DC, that’s convertible, but about high voltage and low voltage. 12 (or 24 or even 48) requires huge currents to get high power. Ohm’s law (V=IR) and power law (P=VI) give P=I^2R , ie heating (and therefore losses and wire sizes) scales hugely the lower the voltage. So to have any opportunity to use your smallish 5kW genset to capacity while it’s running , you need ~400A of current running at 12VDC! The wires would be the size of my arm and very hard to wind into a coil. That’s why those gensets don’t exist. The same is true of the consumers. A 2 hp refrigeration compressor at 12VDC would draw 150A => doesn’t exist.

6. So we don’t need high voltage AC, but to be able to absorb the plentiful power of even small running diesel QUICKLY (so we don’t need to run it for very long), we either need half a boat full of lead, and a very serious bank of battery chargers (as I mentioned above, I think your 3x40A I suspect are probably not loading your genset enough, especially given your refrigeration compressor is 12VDC). Or we need some consumers which can use the power when it’#s produced.

7. the obvious candidates for high power consumers which can do all their work in an hour per day, is refrigeration into holding plates and the water maker (diving compressor for those so inclined). So if we want any of those, and conclude we need a genset (as per your excellent article “Do I need a genset”), then we should put them on 240VAC (or 110 if you must) to able to get bigger consumers which can do the work faster without needing silly size wires. And we should invest in at least 2 100A battery chargers like this: for the bulk charge stage (and do the rest with solar/wind/hydro).

8. And because we don’t need GigaWatts of power and loading gensets is hard, we choose something flexible, small, light and quiet, which retains the 240VAC wire size advantage: – variable speed AC. Given 200A of battery charging @12DC ~ 2.5-3kW plus a big fridge/freezer compressor at 1.5-2kW = fully loaded.

It’s a good discussion, probably deserving of separate article.


Dick Stevenson

Hi John,
As one who has gone down the DC generator path, I have kept an eye on the field. A new DC genset to me popped up a year ago which I have been exploring ( I have seen it at a boat show and liked what I saw. The web site makes the argument for DC generators on cruising boats in just the way I have been writing about for years. The principle, John, has answered a lot of my questions. The unit (actually they have a couple) checks a lot of boxes, but I was unable to find any independent field reports of their use in everyday cruising life (still searching) and without that independent confirmation of its functioning, I would not take the plunge.
I write this as you mention an anticipated article in this area and I believe the ZRD web site and its generators would be of interest. I believe the ZRD concept and design are absolutely what many cruising boats are looking for: direct charge of the batteries in a smaller lighter package than AC generators: plus other benefits.
I just await field reports: perhaps one of AAC’s readers has own or knows of one.
My best, Dick Stevenson, s/v Alchemy


Thank you


When redesigning our electrical system during a big refit a couple of years ago we took a different approach from that outlined by John or its more common variant. Rather than estimating loads and creating a generating capacity to meet them we chose to define the inconvenience we were prepared to put up with for the (electrical) convenience that it would provide. By inconvenience I mean noise, heat, added maintenance, pollution, expense (both initial and ongoing), ugliness and adverse effect on sailing performance.

Since we use run engine sparingly — on a recent 6 month run from California to Tahiti via the Marquesas and Tuamotus we put 8 hours on the engine — using it as a primary generating source was out. Similarly we did not consider being shipmates with a dedicated gas (petrol) or diesel generator. While we regularly make long passages we also spend a lot of time on the hook so a hydro generator would not work. A wind turbine was seriously considered but rejected because of noise and vibration and on aesthetic grounds. That left us with solar, fortunately a good match for us, sailing as we do for the most part between 40 degrees north and south and having a background in land based solar installations in California.

Everything is a compromise and festooning a sail boat with solar modules rarely improves their looks. Our choice was to add a SS stern arch, designed to be as discreet as possible, to carry two 60 cell modules (255 W x 30 V at max power). The location minimizes shading, the bane of sail boat mounted solar, and each module has its own charge controller further reducing the effects of partial shading of the array.

During the same refit we changed over from hank-on head sails to a roller furler. This freed up what had been a large sail locker in the fore peak and we decided, after 28 years, to use it to install refrigeration. After crunching the numbers we built a 2.8 cubic foot top opening freezer with a minimum of 4″ of rigid polyisocyanurate foam insulation on all 6 sides. We installed the smallest air-cooled Sea Frost system with electronic controls which allows us to regulate the compressor’s speed, and thus consumption. Other loads include the usual (LED) lights, moderate computer use, occasional fans and a 500 W sine wave inverter used for sewing projects and to power a blender. Equally important is what we do not have on board. We have no electric pumps of any sort (we use foot pumps for fresh and sea water, have a bilge alarm but no electric bilge pump, a garden sprayer with shower nozzle for showers), no water maker, no wash down spray, no radar or chart plotter and no auto pilot.

The auto pilot, of course, is the big one. We are fortunate to have a boat (45′, 11.5 tons) that can be controlled on all points of sail and at speeds from 1 to 9 knots by a (pendulum servo) vane gear. Since we very rarely motor offshore we have no need for an auto pilot.

How has this design worked out in practice? After 2 six month cruises we are pleased. We keep the freezer at around -10 C (15 degrees F) and have found that 2.8 cu. ft. can store an astonishing quantity of food if carefully packed. Every day or so we rotate ice packs from the freezer to an upholstered high quality cooler that serves as additional seating at the head of the saloon table. This is our ‘fridge and stays at 55 to 60 degrees. Once frozen down the freezer compressor consumes about 3.0 A with a 33% duty cycle ( it uses about 50% more when starting with unfrozen contents), in the region of 25 AH per day. On most days the batteries are fully charged before 11AM and in two 6 month cruises I can count the days when we have not achieved full charge on one hand. Given that it is tempting to think that we could comfortably install more loads. We have found, however, that the line between apparently having plenty of power and finding oneself short is razor thin, at least with renewables.

A couple of other wrinkles that work for us: we use a ridged cast iron pan (readily available in hardware stores, at least in the US, and doubtless on line everywhere) on the stove top to make perfect toast every morning. After years of trying and failing miserably to find a decent cordless vacuum we chanced upon one made by Makita in their 18 V range of tools. As we have a wide range of those tools already we gave it a try…and it really works! We use a 12 charger in the afternoons when the ship’s batteries are already full.

A word of warning to the “we’re running the engine anyway so it’s free power” school of thought. No question this is sometimes true but I can’t count the number of times I’ve asked friends why they motored across the lagoon (or equivalent) in flat water and a lovely 8 knot breeze to be told that they had to pull down the freezer,run the water maker or charge the batteries anyway so they might as well motor. Here, surely, the tail is wagging the dog!

The point I’m trying to make is that not everyone needs a radar, a chart plotter, a forward looking sonar or even refrigeration. If we’re lucky enough to be in a position to design our own electrical system we can balance the convenience of electrical power against the inconvenience of generating it. And that convenience, especially for full time cruisers for whom, in contrast to shore life, the challenge is to find enough to do rather than the opposite, can be vastly overrated.

Oliver Schonrock

HI John

Sorry, Replying out of sequence – 4 levels…

I agree we took a divergence via the esoteric (Elja , Integrel and things that don’t even exist)…however my most recent post does not propose anything that’s not off the shelf and quite proven (deliberately not mentioning brands except for the Fisher Panda, because I am not aware of a direct competitor):

– Fisher Panda 4kW variable speed small genset

– 2x 100A Battery chargers – dump it in quick during bulk, rest with solar etc

– 1.5kW 240VAC 2HP fridgde compressor into holding plates ( we probably need to accept that we need dual circuit holding plates and a run a 5A 12VDC compressor through the second circuit => somewhat higher complexity, but off the shelf and proven).

– Optionally 240VAC water maker (use power when we can).

That’s all bulk standard, proven and nice and modular is it not?


Oliver Schonrock

Hi john

Ok, if you have doubts about variable speed Fisher Panda gensets (fair enough), and you have carefully designed the system – similar to above – to ensure that you can load the genset properly, then a bulk standard Northern Lights 5KW genset will do it too, (variable speed not required – the approach is not reliant on that)

… and it will be fully loaded because of the high power 240VAC consumers (which are much easier to obtain than high power 12VDC ones due to the unsolvable wire size Physics).

And yes 2HP 240VAC compressors are totally standard – think bait freezers on fishing boats and google provides answers very quickly. In fact they are much more standard than 1/2 HP 12DC ones – for all the same reasons. 240VAC (or 110V) is what the world’s industry runs on – 12VDC comes from automotive and unfortunately we are stuck with it on boats, as you said yourself. Your 1/2 HP compressor might be enough at 45N, but I have found it to be not enough at 28Deg Celsius Air and Water temperature @ 10S latitude – assuming 1 Hour run time per day max, which is my personal tolerance threshhold for a thumping diesel genset.

I agree, and have mentioned above, that this discussion is beyond a comment thread. So I will be very interested to read said future article, especially, if you intend to rely on 12VDC and solve the large lead block problem.

Good discussion. Thank you.


Oliver Schonrock

Hi John

It’s good to hear that you made that work in tropics. Yes insulation and condenser are critical.

To be honest my motivation in sharing different ideas was reading your battery articles.

I felt your pain. In there you managed to make some great analysis, great explanations of what is a confusing area for many cruisers. Great testing and superb techniques to extend the life of the bank. I learned some neat tricks from those articles, and appreciate you sharing your experience. The end solution is clever, and obviously effective. It does still contain quite some pain though. Specifically the 2 days per month on shore-power venting the boat and batteries during desulphation. Just not an option for any of the situations I was involved with, because that was all in the South Pacific. And the battery system suffered just like you described.

To be honest it reminded me of painful experiences on several yachts. The constraint in the end – after often 2 years of improvements on each boat – was always the battery system in combination with refrigeration. So my instinct is to “do less” with lead acid and consider the refrigeration as a target to take off the battery load as much as possible. The only boats I worked on, which had systems that were satisfactorily solvable in the medium to long term where those with no, or only modest, refrigeration or those which had that monster you hate (and I do too, to be honest) – the engine driven refrigeration compressor. Many boats had gensets and they did slightly better, but remained unsolved for me. The glowing exception was a boat with the dual high/power 240VAC and low power 12VDC dual circuit refrigeration system.

If your 12VDC based system is a solved problem for you, then well done. I take my hat off to you. Maybe there is a trick I am missing or maybe there is technological advance that was not available.

Very happy to leave it here. More ideas are always Good. Good discussion is healthy.

All the best


Dick Stevenson

Hi John,
Agreed. I thought it was interesting that someone was still out there trying. It can’t be rocket science: a small Kubota diesel with a big alternator. But I guess the devil is in the details and I must wait for field experience to prove one of the attempts successful.
On a practical level, there is a lower limit in boat size for the vast majority of the reliable AC gensets, which leaves a lot of us searching.
My best, Dick

George Meinke

Hi John
Freezer Power Estimates
You can lower freezer power consumption about 40% by keeping it a bit warmer. Set is at 12 -15 degrees Fahrenheit versus 0.
Discussion: A few years ago I wanted to add a small dedicated freezer to my boat, but I worried about having enough power to run it. We spend long periods at anchor and live off the solar panels; the water maker and our large Adler Baubour refrig with its freezer compartment are our big “at anchor” electrical loads.
So I build a 2.5 cubic foot standalone freezer mock-up in my garage using 6” of pink foam to measure electrical load. I used a standard Isotherm compressor unit with an evaporator. Using this mock-up I gathered electrical power consumption data at various temperatures, -5, 0, 10, 15, and 20, degrees Fahrenheit. The garage temperature was about 80 degrees. What I discovered is that a freezer set at 0 degrees F uses about twice the amp hours as a freezer set at 15 degrees Fahrenheit. I could “afford” the amp hours at 15 degrees but not at 0. So is 15 degrees cold enough?
I did a bunch of internet research, the US FDA/USDA guidance is absolute – food will last forever at 0 degrees F; However, there doesn’t seem to be any supporting test data anywhere on the web which provides safe food storage periods for different foods at different temperatures. So….What about -10 or +10 degrees? For the last eleven years, have lived on our boat ½ the year. We have always eaten food out of our Adler Barbour refrigerator freezer compartment, which is between 10 and 17 degrees, depending on where it is measured. We have experienced no food health problems, and the food at 17 degrees is rock hard. I have since installed a standalone freezer and we have it set at 12 degrees Fahrenheit, we have used it for a couple of years w/o problem. Having a freezer, even a small one, is great.
SV Delicia

Thomas Ripple

I’ve noticed that it looks like cell B27 (the required bank size for a lithium bank) uses the usage value in cell B22 in it’s formula to calculate the required bank size instead of the value in B23 like B26 (the required bank size for a lithium bank). Is there a reason that you don’t use the same usage value for lead-acid and lithium batteries?

Peter Herrman

We’ll be using one of these soon.

Peter Herrman

It’s actually on a 40′ workboat RIB, not a sailing liveaboard, and someone far smarter than me spec’d it out. However, we are in the process of buying a Seawind 1260 (2024) and are planning to go lithium (800ah), high-output alternators (, digital switching, lots of solar, and no generator. I have zero desire to have a generator, and I think lithium is only going to get better and cheaper, same with solar. I am trying to get smarter on all this so I can spec it out intelligently. The standard options are not always directed at real liveaboard, bluewater sailors, which is what we want to do. Seawind is apparently pretty amenable to working with owners on non-standard options though. Open to suggestions! Thanks,

Peter Herrman

I haven’t asked them yet, but the only option right now is 12V/120V 60hz, or 24V/230V 50hz. We’re U.S., so I’d prefer to stay with 60hz, but I haven’t thought yet about the ramifications of possibly going 50hz. You can option the boat w/ up to 1200w solar, but I suspect by the time we’re under construction that will be up a bit more.

Peter Herrman

Thanks, I appreciate it. I’ll ask.

Paul Kanev

Hi John,
My Hinckley sou’wester 51/deep keel has a 30 year old greunert AR12 refrigeration/freezer system
In the cool waters of Scotland and the Solent, 1-1.5 hours once to twice per day keeps cold temperatures
As you know parts are difficult to come by in the US and near impossible in UK my 12 volt battery system is nearly 1100 amp hours and the yacht has a panda generator aboard
I consider upgrading to an evaporator system. Certainly lower battery demands per hour but running more hours per day
Would there be any sense or logic to installing a new evaporator system and leaving the older holding plate system in place
This would cost some space in the fridge, not so much in the giant freezer. And redundancy with a working spare/backup in place
Many thanks for your thoughts. I’m a great believer in backup for critical systems (autopilot, comms) but don’t have perspective to appreciate when enough is enough

Paul Kanev

Many thanks for your recommendation. It also brings a weight reduction which is never a bad dividend

Euan Belson

Hi John et al.
As a relatively new cruiser researching the next move, replacing temporary lower capacity GEL, with higher capacity LiPO, the obvious question is the methodology on which to base a capacity sizing decision.
Your ‘books’ and discussions here have blown my knowledge base into orbit. Thanks to all.

Using , (JH), approximation method and comparing with the minute approach offered by some who love big spreadsheets, I have been able to compare the two and on most power drain categories they match up pretty well. Thanks too to Eric, and his power generation considerations. However, there is a term used throughout: <“balancing” battery storage with generation>, which I feel needs qualifying more clearly, and I bet JH will refer me to something I have missed? Please can you help me here.

The other point is the benefit of personal experience and an inability to really track power in and out without the benefit of data from a shunt/reporting system. For us our only significant experience of running short of stored power indicating inadequate battery banks was 4 years ago. With 300a/h AGMS and 1000W solar, supporting a Cairns to Sydney trip c.1000nm in just under a week continuous summer sailing only, dropped battery voltage levels on one dawn, to below 12.4v. Conclusion: there was a cumulative loss of stored power over time (approx 4days in).

I feel there is huge benefit to put together a feel for the power demands and recovery from experience – in our case blunt stick volt meter monitoring and DC-DC displays reporting charging patterns. The key questions therefore are: what are the indicators of low power situations on a basic set up? What sort of conditions over what time period have led to reporting low stored power? Am I off track here?

I might add, I have never learnt more about conserving power than my old household Wattson Power meter sitting atop the TV, which had a large rolling display of volts and watts and net loss or gain indicated by its glowing box glowing different colours.. blushing deep purple for massive draws and bright green for punches of incoming power. I learnt rapidly how to live! (Cut down on the washing).