The Offshore Voyaging Reference Site

Battery Bank Size and Generator Run Time, A Case Study

I recently had lunch with a friend named Hans who owns a very nice 54-foot offshore sailboat. Hans does some pretty serious miles, with a bunch of trans-Atlantics and countless voyages from Nova Scotia to the Caribbean in the fall, and back home in the spring, to his name.

Hans shared that he was considering replacing the house bank on his boat with lithium batteries to dramatically increase the effective bank size from its already large capacity, about 800 amp hours at 24 volts (1600 amp hours at 12 volts).

When I asked why he would take this expensive step, he shared that his goal was to reduce generator run time from its current several hours a day when sailing offshore. (Hans has two AC generators, one of 3.8 kw and the other a monster 14 kw.)


My first thought was that increasing the bank size would not decrease generator run time since the amp hours used have to be put back eventually, regardless of bank size.

OK, I Get It

But then I started to see his logic for his typical four to six-day passages: Nova Scotia to Bermuda and Bermuda to the Caribbean, and return.

In this case a massive house bank (lithium or not) would increase the chances that he would not have to disrupt his sailing with long and frequent generator runs since:

  • He usually goes alongside at the beginning and end of a passage and so can be fully charged before leaving and then recharge from shore power on arrival.
  • These passages typically, or at least often, include some motoring with the attendant battery charging.

And, by going with lithium, the possibility of several days of noise-free sailing goes up substantially, since the house bank could be safely discharged to about 25% of capacity, rather than the 50% that is prudent with lead acid.

We didn’t get into the details of Hans’ usage at sea, but knowing that his boat is fully tricked out with all the gadgets—plus autopilot, watermaker and big freezer—and based on our own usage at sea, I would estimate about 200 amp hours at 24 volts each day (400 amp hours at 12 volts) and certainly no less than 150 amp hours at 24 volts.

So, for the sake of discussion, let’s assume the larger usage and that Hans installs a 1300 amp, 24 volt lithium bank. That would supply about five full days of sailing, based on the above, before he needed to run the generator or plug in—problem solved.

(A lead acid bank of any type—liquid filled, Gell, or AGM—would be impractical, since to have the same effective capacity it would have to be over 2500 amp hours at 24 volts or 5000 amp hours at 12 volts!)

We Gotta Think

Hans’ story is interesting because it illustrates how each of us needs to really think about our usage profile before selecting battery type and capacity, not just:

  • Make a decision based on some rule of thumb,
  • or what someone else has done,
  • or just assume that a bigger bank is always better.

For example, this change would make little sense for a sailing cruiser trying to reduce generator (or main engine for charging) run time, who:

  • Spends a lot of time at anchor and rarely goes alongside.
  • Regularly makes longer passages of a week or more in the trade wind belts where motoring is much rarer.

In both these cases a massive bank (lithium or lead acid) wouldn’t help since the generator (or engine for charging) run time would remain the same, all that would change is the interval between run times.

On the other hand, massive banks can make good sense for motorboats (or sailboats that motor a lot) who wish to remain quiet when at anchor and then will have lots of charging time while moving to the next anchorage.

Big Money

I digress, back to Hans. Of course, all of this assumes that he is willing to spend the some US$10,000 to US$15,000 that such a lithium bank and associated modifications of the boat’s electrical system would cost, not to speak of the added complications and fragility of lithium. (See Further Reading.) None of which he was keen on.

A Simple and Cheap Solution

Given these drawbacks, we chatted a bit more about other ways to solve his problem, during which I asked:

How about if you could reduce generator run time to half or even a third of what it is now for less than 10% of the cost of lithium and with minimal complication and installation hassles?

Hans perked up and allowed how that would be a perfectly acceptable solution.

A Quiz

So here’s a quiz for you: What is my simple and relatively cheap solution?

Read on to get the answer:

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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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Bill Balme

We increased our charging capacity for our 800AH (12V) bank from one 60A charger by adding a 100A Mastervolt and the benefits were immediate. However, I still get very frustrated by the rapid decrease in charge rate. About the maximum charge seen is only about 120A and that’s quickly down to 90A (within like 5 minutes) and then slowly decreases further (as measured on the voltage meter at the panel). I guess this is normal – but I’d love to know how I could fool the system into maintaining the higher charge rate for longer. Any ideas? We typically charge until rate is down to 40A and then let the wind and solar take over – takes between 30 and 60 minutes – enough time to make or heat water too.

Bill Balme

Thanks for the comments John. I actually think I’m being badly guided by a battery monitor that doesn’t know how to count (!) and I’m not discharging to such an extent… Probably only to 70% or thereabouts. The batteries are new-ish (18mo) routinely equalized and I can’t see anything obviously wrong with the wiring. Currently tied to dock power for a month (what a luxury!) but after that we’ll do a proper discharge and see how things make out.
Do I degrade the batteries by NOT discharging to 50% from time to time?


Another great article that gets at the heart of a lot of battery charging issues.
I recently bought a “converter/charger” and from powermax (linked below) and have been really impressed with it as it solves a lot of cruiser’s issues!
There are various pros/cons, and it requires a understanding of your system and the pros/cons, so it is not a set it and forget it, but for an moderately astute cruiser, in the right environment*(see below), I think fills a HUGE need for a budget and huge output charging system. First off, the con, is it is “stupid”, in that there is no automatic temperature regulation. BUT, there is a manual adjustable potentiometer, to set the output voltage. So, in use, you can turn off the 3-stage charging and set it to a constant, but adjustable voltage output and can adjust the voltage that it outputs according to temperature. In essence, it is a manual voltage regulation. The key here, is that in the tropics, the battery is always at or above 70F degrees and when charging will rise to say, 100F, so the charge voltage for a lead-acid goes from 14.4 to 14.05, not a huge swing, and I have talked with two battery manf (lifeline and interstate) and the experts there said that for short charing times, say an hour or two, that the sensitivity to these charge voltages is quite little, so setting it to say 14.2, is sufficient to charge with minimal detrimental effects. Also, since the current through the battery is reduced due to the increasing internal resistance of the battery toward the end of the bulk charge, the heat created due to the high charge rate decreases, thereby self-regulating the temperature to some degree. If the batteries are old, setting a slightly lower voltage will slow the charge rate as well if temp is an issue. The first pro, is that these 120 amp models are under 175 dollars, so I got two, and can run in parallel, charging at just about 240 amps, for under 400 bucks (minus the wiring cost). The beauty is also, two gives redundancy, and ability to load share, when say you want to run a watermaker or other, only charge with one charger.

Steve Dashew described a similar process on Beowulf whereby the alternator regulators were manually adjusted and he would adjust them if he was powering for long periods or charging at anchor or equalizing. He could quickly charge at anchor with high voltage, back it down lower for long motorings, or increase it higher for equalizing charges

This brings up the secondary and tertiary benefits of this charger! Easy use for Equalizing and for long-term battery charge maintenance. This charger will easily increase to the higher voltages required to equalize (upto 16.5 volts, which is sufficient for room-temp batteries) and so it is easy to equalize them without additional hardware.
Finally, long term battery charging, such as when you are on shore power for a month or two, you wont boil all of the liquid out of your batteries like other chargers will. Especially if something like a fridge or other item is left on, which other chargers will get tricked into going into a bulk charge often, with the associated, water boiling, battery damaging, higher voltages. With this charger, you can set the voltage lower, say 13.2, and it will maintain 13.2 volts regardless of the load, and therefore not overcharge the batteries. *Again this could be application specific, as we are in the tropics, and therefore have air con on, thereby maintaining the battery temperature, allowing us to set a constant 13.2 volts.

*(note: I think this works well in tropics since the temp swing is so little. In the high latitudes, where the battery temps will start at a potentially very low temperature, the temp swing is large, and the automatic temp/voltage regulation may be more critical)

Sorry for the length of this, but this charger has been invaluable to us, and therefore believe this charger could be similarly useful to many other cruisers and will help solve may issues that many cruisers face. Even simply the application of equalizing, I believe it is one of the cheapest products that can be used to equalize batteries easily.
All the best,



I do agree that more charge capacity is better. In Hans case his 11 kW generator will only be happy to get more to do.

When it comes to small generators you have to have some margin. They can and should not run at full load for long periods. Do not put more than 75 % load on them. I have a 3600W generator that should be able to produce 13A on 220V. If I try that it will overheat and/or carbon up. It is happy when I reduce the maximum load to 8 to 9A which is around 75%.

Bill. I’m not sure what type of batteries you have but think it is quite normal result. It is no magic you can do. They absorb what they can. The only thing you might be able to do is to increase the voltage if the batteries can tolerate higher voltage. How old are your batteries? They might be sulfated which means that they recharge quite fast as the capacity is low. You typical see that voltage climb quite fast.


Larger slow running generators like Northern Light can run harder. But small fast running 3600/3800 RPM generators is another story.

Often it is only a one cylinder motor. It is very much like the same dilemma as with alternators. You would expect the alternators to be able to run at rated output. But reality is somehing ells.

Reed Erskine

Though I have no direct experience with the Fischer-Panda generator, I have seen quite a few of them in repair shops, and have peered into their innards on occasion. They are highly compact, highly engineered, and highly expensive. I have also heard complaints about them.

Years ago I got a Kubota AE300, single cylinder diesel engine on EBay and cobbled together a 12VDC generator with a belt driven 140 amp alternator. Running at a reduced output to increase belt life and go easy on the 7 hp. motor, it kicks out up to about 80 amps in the initial stages of charging my 400+ amp/hr AGM battery bank. I run it for 70 minutes at a time, twice a day to replace about 100 amp/hr of usage per day. It consumes about one pint of diesel per hr. direct from the main fuel tanks.

It’s been good on long passages (transatlantic, Bermuda, etc.) for overnight charging. My boat has a narrow stern that is already too crowded with outboard storage, radar post, etc. to accommodate solar or wind charging, neither of which work anywhere, any time like the 12V Kubota driven charger. Installed in the aft lazarette, it’s controlled by a series of 12V relays with a timed relay that automates run time, which means that the genset will turn itself off after a set run time, a nice feature when at anchor and leaving the boat charging while going ashore. It probably isn’t as quiet as the Fischer Panda, but we find its industrious hum reassuring. The horizontal, counter-balanced engine design produces very little vibration.

Dick Stevenson

Hi John,
DC gensets just make sense for many cruising boats, especially if their AC needs can be covered by an inverter.
I went that route with an Ample Power Genie more than a decade ago and I have been following the field of DC gensets pretty closely over this time. I still consider a DC genset the wisest way to power a cruising sailboat.
That said, I know of none I would now recommend (see p0ossibility below). Mine is an Ample Power product and has been a continual headache (especially dealing with the company), although I continue to make it work for me and it meets my cruising power needs.
I have never owned an FP but have talked with many owners and seen many installations. Fisher Pandas do look very attractive and (in my experience) fit the nursery rhyme “When they are good they are very very good, but when they are bad they are horrid.” Unfortunately, most/all seem to get to the horrid stage and then have a cascading series of problems from then on till gotten rid of. I would have bought an FP, but I have found just way too many problems from owners, many of whom have turned their FP GS’s into mooring weights: this after spending one or two times their initial investment in repairs etc. and gobs of time in ports waiting for help. Part of the problem are they are so impeccably and compactly designed that they are almost impossible for the average bloke (cruiser) to work on and experienced mechanics do not want much to do with them (they are beautiful to look at, like lifting the lid on a BMW). So, for me, they fail the reliability test.
There have been a handful of DC gensets that have come out over the decades, some connected to dive compressors, others to watermakers, but none I know of have passed the test of time.
It seems like such a no-brainer to make a DC genset (just attach a Kubota small diesel to a big alternator) that I keep looking. It may exist in just this form in a ZRD ZDCGE1222 (, but I am still doing research and would appreciate any feedback, especially those who may have had contact with this Florida based company. The unit looks good on paper and looks good in person at the one boat show where I first saw it. I find the web site to be one of the best written arguments for a DC genset and to cover many other bases but I find little track record for their products or their customers.
My best, Dick Stevenson, s/v Alchemy

marcus petraska

are you able to provide any further insight as to the problems that were occurring with the FischerPanda units – was it the engines or the bits hanging off them?

looking at FP website, it appears they use the small Kubota 2 and 3cyl diesels as a base for their DC units

Dick Stevenson

Hi Marcus,
Good questions begetting partial answers.
On my Ample unit it was the black boxes and “the marinizing” and the safety shut-offs that caused most of my problems. That and a poorly designed exhaust elbow.
Most/all of the DC gensets I am aware of do use the small Kubota engines (mine is a 1 cylinder, some are 2 cyl), originally designed (I believe) to be small tractor engines. It should not be a surprise then that some troubles come with the switch over from air-cooled radiator cooling to a raw water/fresh water-cooled system. The FP I am most recently familiar with had a break/leak spray hot salt water inside his soundproof container and you can well imagine the problems cascading from that one mishap. (As an aside, this seems one large downside of an enclosure.) The rest had a multitude of issues with no common theme. I believe all owners felt that dealing with the FP company was very difficult. I suspect that there is a lot of info on the web.
So, I think absolutely, that it is the bits hanging off them that cause the reliability issues. But that is little different than many/most propulsion engines for recreational boats. The basic engine is almost always solid (certainly the Kubota engine is) and was very likely initially designed for a purpose other than pushing boats around.
My best, Dick Stevenson, s/v Alchemy

Reed Erskine

I second your opinion of the exhaust elbow as a weak point. My most recent exhaust elbow came from who make, or have made, an FP-like genset. Their exhaust elbow seems to be holding up. The combination of corrosion at the welds, and vibration destroys many elbows. The one cylinder Kubota is simple enough to rebuild on the aft deck of a small boat without many tools (my 2011 winter project). Given a good fuel and raw water supply, the little Kubota runs unperturbed in all conditions and angles of heel at 1800 rpm.

YThomas Nygaard

For reliable DC gensets check out Polar Power. Their units are very simple using a robust permanent magnet alternator. They primarily sell to military and telecom applications but have a marine series too based on Volvo Penta engines.

Dick Stevenson

Hi YThomas,
Thanks for your information. Can you site ways of confirming reliability? Do you have first hand knowledge?
Thanks, Dick

Dick Stevenson

Hi Reed,
Nice report and a nice job of cobbling together a DC genset. Please go into the business!
When I hear something I consider unwise to boat or person, I mention it and cross my fingers that one hears it as helpful and not being a busybody. I have come across 3 boats that burned to the waterline and all the fires were on a boat left un-attended with the generator running. Clearly, the good side was that there were no injuries to people, but it was a tragedy nonetheless. I believe gensets, even with the safety turn-offs and the black boxes to monitor the unit need to have someone around to monitor and listen for alarms or other sorts of trouble.
My best, Dick Stevenson, s/v Alchemy

Reed Erskine

Agreed, sailing is a sport that demands a degree of productive paranoia.

Dick Stevenson

Hi Reed,
I like your “productive paranoia” phrase; kind captures an important head-set. Dick


Good morning! A reader pointed out that we had amps and amp hours confused (thanks, Bill!) and I must take responsibility for that. John does know the difference but, when editing, I went for neatness…a definite failing of mine! Sorry for any confusion. All the best, Phyllis


Lifeline have an excellent technical manual. On page 20 you find a formula that can be used to see how much gain you get by increasing charge capacity or increasing battery capacity.


The link for “Why I don’t like inverter/chargers” goes to the wrong place.


John, just a question related to this topic – what do you think of a Dynawatt generator replacing the engine alternator? This would give you 5kW AC when running the engine which could be directly fed into the AC/DC chargers.

Thomas Nygaard

You’re right, it is completely ridiculous to see how many boats have AC generators with a far too small chargers. So of course adding a larger charger helps a lot, but its still silly to generate AC just to have to convert it to DC when you can make DC directly. Ref my comment minutes ago on your previous post, showing how I do that with two Prestolite 140A 24V alternators right on my engine. But there are very reliable DC generators out there, check out Polar Power who primarily sells to military and telecom applications but have a marine series too,


I have a Fischer Panda AGT 2500 diesel DC generator. It went out of production about 10 years ago, meaning this information has mostly theoretical value. It mention it anyway in case there is something to learn from it.
It is tiny even in its GRP and lead sound enclosure and weighs under 50kgs with the enclosure but excluding the external components – see below. The container is only a little over a foot high so it can be installed in the lazarette below the cockpit sole. There is really no other place in our 45” Jeanneau mass production sailboat for a generator and as there is no other generator of any kind that I know of with such little height, this means that when mine dies – most likely because spares are no longer available – that will have been it for generators on our boat.
What I really like about its design is that the sea water pump, the fresh water pump and the fuel pump are electrical units mounted externally (outside the box). The parts are standard items that can be and in my case have been replaced with off the shelf replacements (like Jabsco and Johnson). This generator model has no belt at all as there is nothing driven by the engine other than the injector pump, the oil pump and the alternator. Even the heat exchanger is external – but the “used” sea water is routed inside the generator box to be injected into the exhaust.
Voltage regulation is single voltage via an external “VCS” box. The voltage can be adjusted to any value via a spindle potentiometer – I set it to 14.5V which I measured across the battery terminals.
If I were to design a boat generator, that’s how I would do it. Apparently though, I am the only one as no other generator I have heard of is designed like this.
When running a big load off it, like hairdryer via the inverter, that is suddenly stopped, the voltage can momentarily go as high as 16V until it has throttled down. This can give problems with some electrical components like my Webasto heater but overvoltage protection is a good thing anyway and, since I know this behavior, it’s easy to step down large inverter loads before switching them off.
It is running reliably but has given a number of problems in the past. The Kubota engine has been rebuilt once and it needed a new injector pump. The manager of a Hamburg engine shop must have taken a liking in me and it is no doubt only because of him that my little Fischer Panda is still running. A long-running problem were low oil pressure alarm shutdowns just after startup. The engine shop manager finally traced this down to the wrong oil being used with too high viscosity. The oil I had used was per operating instructions but now that I use lower viscosity the problems are gone.
So overall I must confirm the reliability problems reported above. But I am also continually amazed as to why anyone would install an AC generator in their boats – and even ones sized 10KW or higher (that’s cruise ship-size!).
The max output of my tiny Fisher Panda is about 95 amps at 12V. That is fully sufficient for my 600Ah of AGM batteries as it drops down from 95A after only a handful of minutes usually (I is rare that I have discharged the 600Ah house bank by more than 100Ah).
I have an unusual installation which was suggested to me by a Fisher Panda salesperson. All hoses and cables going to and from the unit including the exhaust are about 5 foot longer than needed so that I can remove the four bolts holding it down, lift it up through the cockpit sole access hatch and set it down on the cockpit floor where it can then be run as normal while doing maintenance or troubleshooting. Other Fisher Panda dealers have warned me of exceeding the maximum exhaust hose length and they may be right but I have never had a problem. This setup has saved me countless hours of cursing to do maintenance.
I realize this generator story is may make other generator owners laugh. But 1,000 pounds of 4 cylinder diesel generator with the alternator mounted on it and running with no sound insulation at all is in no way attractive for me, either.
There are certainly a good number of things wrong with Fischer Panda units and anyway, they dropped this design completely (the smallest DC model they now make has internal heat exchanger and fresh water pump – maybe even sea water pump), but when looking at designs only, I think small is beautiful. But then of course, there’s availability and reliability.

Reed Erskine

Sounds like FP had a good little unit when they started out. Being Germans, they had to complicate the design in the name of improving it. Anyone who has had German cars knows what I’m talking about. Being able to lift out the genset from below decks with attached umbilical connections to exhaust, fuel, power, control etc. is ingenious as well as very useful.
Obviously it wasn’t a big unit. The one I built around the old Kubota AE-300 single cylinder diesel starts to strain a bit at initial outputs of 80-90 amps, which drop quickly as the AGMs soak up the amps. I never run my genset more than 70 minutes, which replaces about 40 amp/hrs. Two runs a day almost keeps up with at-anchor needs, of which 95% is refrigeration.


Wayback machine rules! Finally re-found this very enlighting blog from s/y Whisper (2008) about their experience with FP AGT-4000 DC genset:
The salient points are on the closing remarks:

“A “better” DC Generator (for offshore use)

Simplify everything. Use simple, reliable, proven, off-the-shelf technology. Use properly sized and marinized diesel engines, un-modified water pumps, off-the-shelf heat exchangers, proven couplings, heavy duty brand alternators, and multi-stage voltage regulators with robust temperature compensated battery charging. Remove all custom bits, proprietary software, and embedded firmware that can’t be in-field-user-upgraded.

Design for ease of in-the-field preventative maintenance, repair, and servicing. Ease of access to any/all parts that wear or fail. Quick, easy visual checking of all operational aspects without having to remove a 20 lb sound shield daily. Ports and hatches in sound shield, or ideally, a quiet engine requiring no sound shield.

Improve in-the-field monitoring, logging, and alerts capability. Provide gauges or digital readings to look at when idiot lights do shut things down. Discreet facts of problems and symptoms will speed troubleshooting and repair. Four temperature sensors feeding one idiot light is not useful.

Provide a hot-water heater take-off. This is obvious. If a solution is designed primarily for battery charging, and it has an engine that gets hot, why heat water with an AC element through an inverter? Recommending we buy an AC generator for our 90% 12V DC loads just to heat water and make coffee daily, watch TV for a few hours a week, and recharge our 735Ah of batteries daily through a puny 100 amp AC charger is not a good solution either.

Have a better oil filtering / screening system. Especially one that does not require lifting the engine from its internal mounts every 500 hours.

Parts: Inexpensive non-custom parts that are available anywhere in the world.”

Finally, they ended up on replacing FP with a kubota-prestolite-sterling unit from a small manufacturer from down under, that’s currently down under… but, they got decent charging amperage, hot water, and very much more maintenance and purse-friendly unit.
But, they had to troublehoot and re-engineer even the new unit. I think they would have been better off by having rolled their own kubota-prestolite-balmar genset from the start…


So, it seems there’s lack of good turn-key options on DC genset realm.

If requirement is
– at least 240 amps of reliable charging current
– good reliability and easy access while underway
– off-the-shelf spare parts, worldwide availability
– plus hot water,
none of the commercial DC genset offerings deliver. Please correct me if I’m wrong.

Reed Erskine

You may be right, but a two cylinder diesel should make this possible. The big advantage to the single cylinder Kubota diesels was their horizontal design, perfect for a compact below decks installation. All the multi-cyclinder diesels seem to be vertical designs, resulting in awkward sizes.

The immense fly wheel on the single cylinder diesels is what gives them their power, but they don’t like being overloaded. My little Kubota genset engine, the AE 300, has nearly 1000 hours and shows no sign of smoking, carbon build-up or soot darkened sump oil. I tried to sound proof the lazarette in which it is installed, but it couldn’t be called quiet. Diesel aren’t quiet. It emits a not unpleasant hum, audible to neighboring vessels at a range of about 20-30 meters. depending on wind and ambient noise level.

Dick Stevenson

Hi JC,
I do not know of any turn-key DC gensets that are reliable, period. Let alone the other requirements. A couple of thoughts:
I suspect shooting for 240 amps may be too ambitious as it means a big engine and real estate issues. My Kubota AE300 (1 cyl) started out putting out 140+ amps (150 amp alternator with a separate rectifier) and after 12 years of use (and more than occasional abuse and mishaps) is putting out 110 at the charging outset. These small Kubotas are good engines, but primitive in their overall functioning with the change-over to raw water cooling etc. They take a while to get to operating temp. I am happy running harder a bit longer and I am still under an hour a day in the cold waters I am in.
Good reliability has been discussed. Does any genset on a small/medium size sailboat every really have good access, especially underway. Mine is adequate and many I have seen are miserable.
Hot water is easy with a hot water heater with two circulating engine coolant circuits (one for the engine and one for the genset). When in a hurry, I can use my inverter to heat water when the GS is going and the batteries are being charged.
My best, Dick


I have an Indel Webasto Marine “Double Coil 40”:
I guess I would call it “good” as it did not develop a leak in the 10 years I have owned it, is all 316 stainless inside, has decent holdover time, has two coils plus electric, has moderate AC power draw of 750W. They have an optional hot water mixer valve that stretches the 40 liter volume by mixing the hot water down to a lower settable temperature with cold water. Also an overpressure and drain valve.
They’re available off the shelf in Germany.

Dick Stevenson

Hi John,
Mine is just what Henning reported. The smallest dual coil is an Isotemp 12 g unit called, I believe, a Basic 40. 12 g is a bit of an overkill for my size boat, but I wanted the dual coil. It is a great unit and I have had good service over the years trouble shooting a pressure relief valve that started dripping 4-5 years ago and this year easily replacing a temp sensor. Mine is about 14-15 years old most of that full time live-aboard.
USA Indel Webasco is 954-984-8448, Great Water (in Maine I think is a distributor, Kathy does tech inquiries and is very helpful) at 774-328-9498.
I believe the Isotemp unit is well insulated in the scheme of these units, but my next install will include insulation around the round shape of the unit to extend its hot water life. My water heater is in a locker surrounded by the 5c/40f water Alchemy sits in and the water definitely does not stay hot as long as it does in warmer waters.
Come back with questions. My best, Dick

Max Shaw

Back in the world of internet for a few hours … I believe Torrid also produces dual coil hot water tanks. Unfortunately their website is not showing any products at the moment so I cannot confirm ( and we are back away from internet tomorrow.

We have a 10 gal 14 year old single coil Torrid in Fluenta. It only just started to have some leaks at the output this year but, with some great tech support from Torrid, we were able replace the plumbing fittings to fix the leaks while we were creating chaos in that part of the boat anyway installing our new hydronic heater system.


SV Fluenta

Gari Williams

John, I’m not an expert in electrics but I can follow a reasonable argument. Your ‘example 1’ above has prompted a response from me because I think it is too much of a generalisation. I suspect your logic has derived from focusing on your particular AC generator setup which is a constant speed unit and uses the generator’s rpm to define the frequency of the AC. As a result it must run at a steady rpm and outputs frequency stable AC. However, the smaller AC units that run at variable rpm cannot do this. As a result I’m pretty sure that they use alternator technology to generate frequency wild AC, it is rectified to DC and then an onboard inverter is used to generate frequency stable AC. If that is the case, then for smaller generators the DC machine has fewer steps in the generation of output power and thus wins the “simpler is better” argument. From my general research ‘smaller generators’ includes most (all?) sub 6kW units.


I think that last point is the main factor in reliability. If Fischer Panda would for some weird reason begin to sell 10’s of thousands of AGT models and do so for a decade, they would come out near-perfectly reliable IMO.
I’m not an engineer but as far as I know, everything that generates electrical current by some form of motion produces AC voltage. My Fischer Panda DC generator has rectifying diodes. I’m not sure they are a source of a unreliability.
Unrectified AC current can be used as is but then the engine driving it must keep very precise RPMs and run at typically 3,000 RPM (can be less, but rarely, depends on the type of winding). If it’s rectified, the RPM can be variable which I consider desirable. Rectified current can be used directly (DC generator) or can be inverted to 110 or 220V AC. The latter is done for example by more modern Fischer Panda “i” models. They even make a model where the inverter is also the battery charger and everything is in the same case – the battery charger vibrating with the engine. Very likely typical German overengineering and to hell with reliability.
But be that as it may. I have been priced out of the market – spacewise. _No_ generator other than the obsolete one I have will fit my boat and this happens to be DC, which is fine with me. I expect that when it breaks I will replace with a fuel cell ( which will fit and I don’t read many bad things about them other than sourcing and storing the fuel will be a bitch. The largest model will do 200Ah/24hrs and so will cover 100% of my at-sea consumption. It sells for about 7,000 US, probably less than what my Fischer Panda has cost in about the year 2000.

Ann B

This post has been very informative. On our ‘new’ boat, we have a Fischer Panda 8000i (8.0 kVA, 6.3 kW, 230VAC) installed by the previous owner in 2015 (base unit cost was NZD 13,325). I can’t speak to all the technical details, but, it has been running reliably for us for a year now. It is extremely quiet and we usually run it once a day at dinnertime to charge batteries using a 220V charger, use the electric oven/cooktop and make/heat water. So far, so good. Only issue has been a shredded alternator belt.

Dick Stevenson

Hi Ann,
Been a while and good to hear from you.
Are you saying that the genset needs to be going for you to be able to use the oven and cooktop? Are there other means to cook/heat water etc without electricity? I know some of the bigger Oysters rely exclusively on the genset going for their electric stove-top/ovens.
Best to Glenn, Dick Stevenson, s/v Alchemy

Ann B

Hi Dick,
We had very big doubts about this setup too, but, it seems to work ok so far and meets our requirements. Without the genset we can run a microwave or one cooktop burner on an inverter. The oven and/or two burners requires the genset. One run of the genset sets up hot water for a couple of days. If the genset packed up completely, we can still cook, just no oven. Hot water could be heated from the engine takeoff (not currently hooked up). For the worst case scenario, we have a backup campstove. In the meantime, it’s nice not to worry about a propane installation, its maintenance requirements and potential dangers, or, having to find a propane source in out of the way places. The oven/cooktop are standard Bosch (non-marine) units (gimballed of course).

Dick Stevenson

Hi Ann,
Sounds like you got it covered. And, for sure, the lack of any consistency on propane attachments is one of the bigger headaches in multi-country cruising and it must be nice to finesse that challenge. Thanks for the info.
My best, Dick

Jamie Abdy Collins

John, I am needing to replace 20 year old Rolls lead acid batteries totalling 1350 amps, there is a further shore set of a further 2 rolls, 486 amps. The starter battery is separate to this lot.
At present there is a 12.5 Iva kubota generator and a 150amp Xantrex 3000 inverter charger.
The boat is a Sundeer 64, hence the weight/ballast consideration.
The yard is suggesting
We propose installing six Victron super cycle AGM batteries at 230ah each. These are the basic size of 8D’s (532x207x218). We would stack them on shelving, 3 in each sump bay. The sump layout only allows space for 6. The total ah for the bank would be 1380 ah with a total weight of 810 lbs. compared to the existing bank of 1725ah and the weight of 1027 lbs. We have space for adding ballast to make up for the weight loss. The thought to make up for the amp hour difference is to change the existing inverter/charger with a Victron Quattro 12/5000 inverter/charger. That would allow 220 amps of charge for the service batteries as well as float charging on the engine start battery. It would include a color control monitor that would monitor batteries, charging (including wind and solar). We also haven’t finished the load calculations yet, but feel the new systems in the boat will actually be less than the original boat load.

If I was to go with this would I be able to add the old xantrex charger in series to the new Victron.

Stuart Jackson

This article was right on point for me and my boat. I have fridge, freezer, autopilot, electric kettle adding up to 250-300 Ah per day. I have a 8Kw generator but only 390Ah of house batteries, which required charging multiple times a day from 60 amp charger. I added a second Skylla 70 amp charger for $800 – a cost effective solution that cuts charging time in half.