The Offshore Voyaging Reference Site

Cruising Boat Electrical System Design, Part 2—Thinking About Systems

In the last chapter we thought about what our electrical consumption was going to be and (hopefully) wrote those numbers down for both sailing offshore and at anchor.

Next we need to add in any other big loads that I did not deal with in the last chapter. Two that come to mind are 12/24 volt DC watermakers and furnace type heaters like those from Eberspächer (Espar) and Webasco.

(If you are wondering why I didn’t mention these two before, the reason is highly technical…I forgot. Sorry).

That said, my senior moment is actually a good thing, because watermakers give us another chance to think about smart management of all loads, so that we can build an electrical system that does the job of supporting happy and comfortable cruising without requiring a really silly number of batteries.

Let’s look at that:

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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—Part 1, BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Part 2, Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Part 3, Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Part 4, Fusing
  34. 11 Steps To Better Lead Acid Battery Life
  35. How Hard Can We Charge Our Lead-Acid Batteries?
  36. How Lead Acid Batteries Get Wrecked and What To Do About It
  37. Equalizing Batteries, The Reality
  38. Renewable Power
  39. Wind Generators
  40. Solar Power
  41. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  42. Battery Monitors, Part 1—Which Type Is Right For You?
  43. Battery Monitors, Part 2—Recommended Unit
  44. Battery Monitors, Part 3—Calibration and Use
  45. Battery Containment—Part 1
  46. Lithium Buyer’s Guide—Budget: High End System
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Eric Klem

Hi John,

Just getting around to processing this and the last article on loads. My initial reaction was along the lines of yikes, I don’t want a boat with those sorts of loads. Thinking about it more I realized that the boats that I have sailed on have widely varying electrical loads and your numbers are unfortunately not unreasonable for a boat that hasn’t been setup with minimizing loads in mind. I have a friend with a 92’er that is repowering right now so we looked a lot at the power budget and his loads for that big boat and up to 32 people aboard are typically under 150 Ah/day in mid summer doing coastal cruising type use (ice instead of refrigeration is a huge difference here). On the other hand, I have delivered boats under 50′ that we had trouble keeping the loads under 500 Ah/day. Our own 36′ boat which I find just as comfortable as some of the 500 Ah/day power hogs is typically 50-60 Ah at anchor and 70-100 Ah/day underway depending on conditions and radar usage but we do not live aboard and are setup for coastal cruising with most hops being under 300 NM so the numbers are not directly applicable to long distance cruising.

The best way to deal with loads is of course with conservation which takes on 2 forms, efficiency and behavior. On the efficiency standpoint, many boat systems are not the most efficient, hydraulics for autopilots being one of the glaring ones and poor fridge insulation another. I actually keep a power budget for our boat and each new item we consider installing is plugged in and viewed as a part of the whole and more than once, this has changed our decision on a piece of gear, for example getting an electric autopilot instead of a hydraulic one. Behavior can be tricky but it is amazing what a difference it can make just doing little things like being organized about getting stuff in and out of the icebox and making sure that it does not stay out long and get warm. I have seen very little information published on how to evaluate the actual load based on specs such as converting COP and insulation to Ah which is unfortunate. I would think that it is reasonable to cruise full time on 200 Ah/day in moderate climates but that would require real effort being put into lowering loads.

The other thing that makes sense is some sort of load balancing and understanding time factors of loading. You have touched on this in your article by suggesting that loads happen while motoring which is probably the biggest thing. Another example might be that in the tropics, you won’t have the heater load but in colder climates, your refrigeration load will be lower. As more renewables get added to energy grids, this field is expanding rapidly and some ideas may apply to boats.

In the early days of off-grid solar and wind applications, people tried really hard to cover all their needs which usually meant designing a system for a cloudy January complete with enormous battery banks and huge solar arrays. These systems were typically operating at a small portion of their capability and would only ever see their design point every couple of years. Since then, people have done a better job of designing for ~90% of the time and making up the difference with temporarily allowing a lower SOC, reducing loads and running small generators. The important lesson there was in figuring out a realistic load as opposed to stacking worst-case assumptions to the point where the design requirements become too difficult to meet in a reasonable way. Figuring out what a good conservative design versus an overly-conservative design is an important part of my job and we don’t always get it right which either results in poor reliability or poor performance.

Looking forward to the next couple of parts.


Oliver Schonrock

Hi John

In our last exchange, you said “we are not going to converge” (on, put simply, low voltage DC vs high voltage AC). I am happy to report that I am coming around to your way of thinking. Perhaps old dogs can learn new tricks?

I said: “Perhaps, I have missed a technological development”, and this may indeed be a factor. After a lot more research into various power hogging drive systems (mainly refrigeration and watermakers) and the motors used for them, I am coming to the conclusion that there have been massive advances in the efficiency of those 12VDC machines compared to to their AC counterparts – particularly for the sort of fractional horsepower level which is typical on boats.

Some of my lead-acid nightmare concerns remain, but the efficiency gains of 12VDC motors are so great that it may indeed make sense to consider operating most of the consumers in that mode (higher voltage DC would be even better, but just not practical on boats). This change of approach has a lot of implications on how to tackle the whole system.

Still thinking and digesting…


Matt Marsh

For the moment, my thinking usually ends up at:
* If you already have a 12 V system, stay with that, and do the best you can with it.
*If you’re starting from scratch or doing a major overhaul, and your loads come to less than about 2 kWh/day (i.e. 166 Ah/day at 12 V), install a conventional 12 V system.
*If you’re starting from scratch or doing a major overhaul, and your loads add up to more than about 2 kWh/day, install a conventional 24 V system. Use 24 V devices wherever possible, and use an engine that’s natively wired for 24 V. You’ll spend a bit on DC-DC converters to run some electronics, but you’ll save more than that in lighter & cheaper wiring, and 24 V motors usually run cooler, last longer, and are more efficient.

Looking forward a few years, the car industry is beginning to change over to 48 V DC, and the higher-end parts of the marine industry will probably follow. There are huge benefits to that. Among other things, it means that 10 kW MGUs (the multi-function motor/generator units that will replace conventional alternators) at 48 V are going to be made in batches of ten million at a time, along with 48 V variable-speed refrigeration compressors and other high-power gadgets. Imagine running 10-gauge instead of 2-gauge cable to your windlass; that’s the change we’re looking at circa 2025.

Eric Klem

Hi Matt and John,

On a very high level I agree with you that 48V makes a lot of sense for exactly the reasons that you state. However, I think that there are a few practical issues that will keep most of us from going to 48V beyond simple part availability for a while yet, at least to Matt’s 2025 prediction. The first is the 2V per cell that you find in lead acid batteries. If you want to use deep cycle such as GC2’s, then you are stuck with multiples of 24 cells which in the GC2 configuration means that you need multiples of 8 batteries. Smaller motors are voltage limited for technical reasons so some equipment may need to gain some electronics to do this. Electronics keeps becoming cheaper and having step-down converters on each appliance may soon become standard and if a newer battery technology tips the scales away from lead acid, then 48V may well become the standard. On the flip side, I would love to run a bunch of small cabling but I don’t find the large cables that people complain about to be nearly as big a deal as most people make it out to be. For the complexity level boat that we have, I think that 12V makes sense right now but for a more complex boat, I would certainly look at 24V and see whether enough items were available at that voltage.

I go through this voltage debate with some regularity for work although we have no batteries, it is all mains powered. By international standards, 48VDC is the highest standard voltage we can run without putting a lot more safeguards on our system. However, we will have some actuators that are small enough that they are only available in 12 or 24V, not a big deal when running a motor controller but something to think about. Also, we often design so that we want the full power available and the easiest way is to run a higher bus voltage and then step it down locally to exactly what we want and focus on voltage drop from there as opposed to over the whole system. On the personal side, the electric cars that I have built were 144, 156 and 12V (electrathon so low voltage) and the hybrid was 120V and you do have to be real careful with those voltages when they are DC, contactors are your friend.


Eric Klem

Hi John,

I look forward to the piece on the new system. I haven’t had a chance to look at it yet so can’t pass any judgement myself other than to say that I am sure it is overkill for the loads I try to strive to maintain.


Per Kjellqvist

Adding on to the watermaker discussion I would like to put my hand up in favour for an AC driven watermaker. Why, well if you already have a large enough generator which needs extra load when in use, a watermaker is a great load.

I built my own watermaker 10 years ago and did a lot of thinking – research – thinking etc. before deciding on an AC pump and as big membranes as I could fit in the boat. Our watermaker produces 120 lit/hour.

Like you we typically run the generator 1h / day. Our loads are 1) battery charger, 2) watermaker, 3) electric oven to make lots of passage food, 4) dive compressor, 5) and if we need more load we might switch on the air-con (and those are the only times we use it).

On days when we need to make more water, we run the generator a bit more, but that is rare. If we are sailing we run the generator for an hour and have some fun using all the power as per above, and if we are motoring I am fine with running the generator as well as the engines for an hour.

We can produce water without having to increase the battery bank while keeping it very simple.