The Offshore Voyaging Reference Site

Renewable Power

People have some strange misconceptions about what you mean when you say that you live on a boat. They seem to either think that you live on baked beans in a floating cave, devoid of light and comfort, or they imagine a miniature cruise liner, with lights ablaze and lobster and chilled champagne on tap.

The truth is a distant somewhere between the two, of course, that depends on the boater. We think we live very well indeed, but we’re both used to living in remote and service free areas. Others would draw the line at our lack of amenities: aircon, washing machine, etc—but we’re happy.

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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
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If you get the chance take a look at Nigel Calder’s work on specific fuel consumption in various charging scenarios. Charging the batteries with the main engine (essentially unloaded) is one of the most expensive things one can do on a boat.

Not to mention not using the main engine to charge batteries is better for the local environment, hearing and psyche!

Colin Speedie

Hi Chris

Running the main engine unloaded to charge batteries is just about the worst thing you can do for many reasons! If you have – repeat, have – to do it when alongside the only answer is to double up your lines and put her into gear to create the load. But what a palaver, let alone the nuisance factor for your neighbours…

Best wishes



Or if at anchor or mooring, put the engine in reverse.

Erik Snel

As we have a much smaller yacht (34 foot), means of generating power have to be fitted in a smaller setting as well. Therefore we only have 100W of solar panels. We decided not to go for wind power as the stern is already packed with solar panel, windvane and radar/antenna mast, leaving little room and to much weigt at the stern as it is…
We therefore decided to look at the consumption side. We made a big reduction there by replacing all internal and nav lighting with LED. Now we can light the whole cabin for less than half an amp. We also installed breakers for each indidual instrument, also reducing the consumption considerably off shore.
Finally, we only run our fridge when motoring or on shore power.

What I am still considering is a prop driven generator for while under sail. The prop has to revolve anyway and it will probably slow us down by 0.2 kn at the most when there is enough wind. However, I am very interested in your experiences with the new hydro generator as an alternative to the prop generator.

As it is, with this setup I haven’t yet had to run the motor to charge the batteries, usually when underway for a longer stretch the moment we decide to start the motor due to lack of wind comes first.



Colin Speedie

Hi Erik

We’ve just antifouled and launched, so we think now will be a good time to look at drag caused when the hydro generator is in action – which will make it into the part on water powered generation. As will prop driven alternators, which I’d agree are a good option for the right boat.

It does sound like you’re doing pretty well as it is – hopefully there’ll be more for you to comment upon as the series progresses.

Best wishes


Larry Vito

Hi Erik,
What kind (brand and model) of LED interior lighting do you have that uses only half an amp?
I would like to know where I can get them.


C. Dan

This is very exciting! Can’t wait to read more.

If possible, I would be curious to see a summary of your “ideal” system under different energy budget scenarios (e.g. with and without freezer… and yes, maybe a small air-con unit, even if it’s just to demonstrate how it makes a system unworkable)

I am also curious how much output you get out of the towed unit, and how much measured degradation of speed there is

Lastly, since I’m emotionally invested in the A-40 project, I would appreciate some indication of cost of installation (beyond the A-40 sticker price) that you contemplate for this system


Colin Speedie

Hi C-Dan

I think it might be hard to come up with definitive answers for particular scenarios, as we’re dealing with so many variables. What might be possible is a ‘ball park’ value for high, medium and low demand craft. But we’ll see…

We haven’t been able to measure the towed unit yet, as we have to fit an in-line ammeter, but hope to soon, certainly before the relevant posting.

And cost, together with my suggestion of an acceptable mix of alternative power supplies will definitely be included for ‘my’ A40 – look forward to hearing what you think.

Best wishes


C. Dan

One other thing that I would appreciate, in the context in the target market of the A-40, is an article/guide for aspiring blue-water cruisers who want to be self-sufficient in electrical systems troubleshooting and installation.

My boat’s electrical system currently consists of a car battery, a 5w trickle charge solar panel, a radio, depth-sounder, and a few lights. For me, the systems that one finds on a typical 40′ cruiser are daunting, if not downright terrifying.

Even a simple list of books would help. I’d also like to hear some examples of how you decide when to turn a given problem over to professionals.

Colin Speedie

Hi C-Dan

Yes, I agree that most boats today (even at 40ft) are over complex, but it’s also the case that many of today’s cruisers don’t have much in the way of basic skills – some I’ve met could hardly change a lightbulb.

Most, if not all, of the options we’ll be looking at could be installed (and serviced) by any averagely competent home mechanic/electrician, perhaps with the exception of any mounting frameworks or the like. This series will concentrate on what’ s available, what’s possible, and what’s easy to live with, in every sense. Let us know what you think as you read it, and watch out for the wealth of great comments we expect from our readership – I think you’ll find much of what you’d like to know along the way.

Best wishes



Hi Collin,
Exciting article and timely.

Tracy and I maybe one of the few who consider breaking away from the addiction of genset, a great positive. No genset is one of the most exciting things about our new boat.

We will go with wind, solar and the prop generator. The builders of our new boat put the prop generator on their personal boat and talking with them recently it sounds like they averaged around 7 amp hours with sailing speeds between 5 and 8 knots. The power curve is much the same as a wind generator.
We plan on about 450 amp hours of batteries which is less than our last boat. We feel confident we can easily keep them close to full except under worst situation at anchor where we have heavy clouds and no wind. If that happens we will pull anchor and go motor for a couple of hours and troll for tuna and Mahi.

What impresses us most is the new technology in things like refrigeration, lighting, radar, chart plotters and the small DC water maker that use just 5 amp hours . The refrigerator with small ice box uses only about 50 amp hours a day and gives us enough ice for evening drinks. We have become great at caning so fish and meat no longer need a freezer. Our last frige/ ice box used 135 amp hours a day in the tropics. And we all know how well LED lights are doing, both interior and mast head lights.

I’m thinking the only time we would have to run the engine on the new boat for power is if our batteries were a bit low while under way and we were speaking on the SSB.

In this day and age we think we can have most of those enjoyable luxuries without the genset.

Looking forward to your ideas and others in this new age of less energy consumption.

Steve and Tracy.

Colin Speedie

Hi Steve and Tracy

I think you’re helping to show the way to less energy dependent boats – and more will follow.

As you say, the multiple small improvements at both the supply and demand end of the electrical equation are making it far more possible to live well and comfortably without a genset. We do.

And there may be a few extra ideas in the proposed pieces to help you fill the ‘energy gap’ when you’re at anchor – let’s see.

Best wishes


Paul Mills

Hi all,

I am really happy to hear people talking about not needing a genset. A couple of weeks ago I met a crew who’s new 47 footer has a genset, after the first 4 months of cruising, with solar, wind and engine in and out of harbour, they told me they had not needed it once …. .

On sakari we have found that our D400 wind generater combined with 250w of tilting panels has met all our needs, especially once the crew are ‘trained’ to tilt the panels in a pavlovian manner. When sailing, apart from downwind in lighter airs, I have found that we can keep up with consumption, if the screen on my plotter is put into sleep mode and we are not charging laptops etc; however when on the wind we never run low. We also have (apart from for motoring) all LED lights.

Having the battrey monitor, with its large display prominantely placed has also helped awareness, and I inwardly grin as I begin to hear my crewmates tutting as current draw goes over 7 amps …, and at other moments commenting that we are still charging at 2 amps – with everything on!. For me, the beauty of wind and solar is that they just do their thing, so we rarely get below 70% – and that’s usually at anchor on a cloudy/wet calm day when we are laptop-ing.

One thing I have in mind to change is the control of our Simrad autopilot. At the moment the GPS has to feed via the plotter, and this means significant extra amps. I beleive that a crossover? switch could enable switching off the plotter, and save considerably. It’s near the top of the list for this winter.

Colin, I am guessing that your towed generater will be a lot less £’s per amp that a prop driven one, and somewhat envy the routine of streaming the impeller – takes me back to the walker log we had on Provi….., mind you I am also recalling seaweed, jelly fish tentacles and the odd person motoring too close astern…..

Looking forward to this series.


Colin Speedie

Hi Paul

All good stuff, especially your point about being able to see the battery monitor – I’ve been told by a friend that installing such devices in the average home would make the greatest difference to reducing power consumption imaginable!

We’re hopeful that the towed generator will deliver – so far the results have been very positive. But the local (and very experienced) electronics wizard says he’ll bet that we don’t make it to the other side of the Atlantic with the original prop – he reckons most get eaten by big fish along the way – which is why we have a spare…..

Best wishes



Hi Colin

I’ve used a towing generator and been very happy with it. Am sure that you will too. I also believe that idea that big fish will eat the propeller is largely a myth, and I’d be willing to take on the bet of your local electronics wizard. 🙂

But what is worth being aware of is having a long enough towing rope. My aqua/aerogen was supplied with a line of less than 14 meters. It meant that in large waves the propeller kept jumping out of the face of a following wave. And when highly twisted rope becomes free, it kinks, and rapidly turns into a giant knot. Fairly soon you will have a huge and messy knot in from of the propeller, which stops it from working. I bought a new line somewhere between 25 and 30 meters long, and it helped a lot. Even then, though, in large following seas between the Canaries and Cape Verde, after several days of force six+ wind, we had the same problem again.

So make sure your towing line is long enough. You might even have to increase the scope still more if the attachment point of the generator is higher. On your Ovni this should be a bit higher than on my relatively low 33-footer (Elan 333).

Colin Speedie

Hi Axel

I hope you’re right! We’ll see if we make it over with the prop intact.

Our unit is the Ampair made generator, which comes with 30m of 12mm line as standard, but with the recommendation that this can be increased to 40-50m of 14mm if necessary. We’ve also got the coarse prop if we’re exceeding 7 knots regularly. Having heard of problems with units mounted too high we had a custom strop made up and fabricated a mounting on our stern arch to take it – as a result our unit is mounted much lower than you’d expect.

Hopefully it will all work as expected…..

Best wishes and thanks for the tips



Our solar panels were stolen from our hard dodger roof in a storm.
Our air marine wind generator was mounted on a meter long pipe welded to the aft end of the port davit, this was cut off at the davit using a thin cut off disk presumably with a battery operated grinder.
This presents a different aspect to the cost analysis of power generation !!

Colin Speedie

Hi John

That’s too bad, and what makes it worse is that you can be almost certain that it’s another sailor who has done it – or bought it cheap, knowing it ‘fell off the back of a lorry’.

I had a friend who had a windvane stolen from the stern of his boat in a busy yard, and ever since I’ve unbolted ours when we’ve left our boat for any length of time.

Might even get worse in the current financial climate – sad, isn’t it.

Best wishes



I find this site very useful and it has influenced the set up of our boat. We have a 1998 Moody 40. We have installed 380 watts of fixed solar panels on an arch, a duo- gen wind/water generator and have a 115 amp alternator. At anchor the solar panels keep us at or near full charge even on a cloudy day. We have a fridge and freezer running full time as well as the usual electronics. The two biggest game changers for our boat are the solar panels and switching all lighting to led’s. The duo-gen in water mode provides 3 to 8 amps when we are under way, provided that we are moving at 4 knots or better. It has the advantage of not requiring a tow rope, so less drag and much easier to recover.


I was consider a freak for not having and wanting a frigde,hot water and pressure water (only foot pump) on our brand new Garcia 46 , but never ever missed this power hungry luxury .
i always liked the sunset hot solar shower watching the tropical landscape better than the claustrophobic heads.
Collins, did you consider the Wat and sea water generator before installing the towing gen? If yes what made you deciding in favor of the towing prop gen

Colin Speedie

Hi Giancarlo

I very much like the look of the Watt and Sea, but (a) they’re very expensive, and (b) we felt it might be more than we need.

We have some friends who have just fitted one on a bigger boat, and we’re looking forward to hearing from them how it performs – which we’ll be sure to pass on.

Best wishes



Ciao Colin,
Sorry for mispelling your name in the previous comment.
Considering the many different options for our new boat equipment i also consider not to install the windvane instead having the Watt and sea generator to feed the autopilot during long passages, i think the hight price for the Watt and Sea may be similar to the price of the windvane and a towing generator,of course the solar pannels will provide more than enought to run the Vesper marine Whatchmate ais and vhf
As usual deciding the right equipment is always a compromise
Before the final decision i will wait until i have a first hand feedback from Watt and Sea owner
Buon vento

Colin Speedie

Hi Giancarlo

I understand your thinking, and I’d agree re the cost, but our view is to have a windvane and an autopilot for a number of reasons – not least that one we’ll always have the back-up of two systems, but also our concerns over the reliability (or lack of) with autopilots.

As I mentioned before, I’m hoping to report on the Watt and Sea from our friends who are very experienced people. I also heard some interesting comments today regarding the adoption of the Watt and Sea amongst the single handed racing fleet – but if you don’t mind, I’ll keep them for the article on hydrogenerators!

Best wishes


Colin Speedie

Hi Ted

Sounds good, and yet again proof that if you start at both ends of the equation it can be done!

Hopefully we’ll come up with some new ideas that may prove useful to you for the future.

Best wishes


Dick Stevenson

Dear Colin,
Good posting and responses.
Some points:
1. In my observation, there are only 2 criteria that scream for a genset; a freezer or aircon. It is a rare boat that can have a freezer (of any reasonable size) without a genset and generally impossible for aircon. In 10 years of full time live-aboard life Alchemy has never wanted aircon. The freezer, however, has been and remains very useful and appreciated, especially when we were off the beaten path for weeks and months at a time. (see below for genset thoughts)
a. That said, our freezer, which is large, is 2/3rds of our daily at anchor power requirement, more so in the tropics.
2. It is my observation that by far the majority of full time cruisers (by that I mean people who have no land base to return to periodically and who wander about) with boats over 40 feet have a genset.
a. No system gives live-aboards more headaches than a genset.
3. Again, it is my observation that most boaters, before they enter live-aboard life, do not have a sense of how a boat functions on the hook for long periods or on long passages. For many who, to take full advantage of their holiday, are used to moving every day or 2; their batteries get charged (and water heated) in the motoring into and out of anchorages or in the regular marinas on shore power. When living aboard, you stay longer in places of interest. I urge those who are designing a system to be on the hook for a week or longer and see what the challenges are for their vessel. Make water a few times during the stay if you have a watermaker. Figure out how you will get hot water if that is something important. Most anchorages are chosen for attributes that preclude much amperage from a wind generator so much of the time it is solar alone. A simple life is fairly easy with solar panels in the Carib or Med.; less so the farther N you wander. As a note, it is my opinion that solar panels should never be attached to lifelines or stanchions on any vessel intending offshore passages. An arch can be ideal, but is not something many want for a variety of reasons. That leaves little alternative space on many vessels.
4. The above may point to a difference in head-set from many AA readers: in most ways Ginger and I do not consider ourselves to be engaged in an adventure. We wander fairly widely under sail and average 2-4000 miles a season. Ours is a lifestyle, not an adventure. As a lifestyle we are in no way interested in what we consider camping. (We love camping, but it is not our chosen lifestyle.) As such we like our freezer, frig, watermaker (not so much in UK waters), computer use, SSB, a well lit cabin at night is cozy, furnace when cold, hot water showers, electric auto-pilot (the Monitor is too fussy for short day hops in near land changeable conditions), etc. etc. It is our home and we like it to feel like a home.
5. One might say, our life style is both an attainable adventure (in part as we keep things smaller than many) and we try for a sustainable adventure. Everyone’s desires, limits and resources differ and a crucial piece of designing the power generating /storage capacities for your vessel is being clear about your (and your partner’s) desires, limits and resources if you want your cruising to be sustainable.
6. About power design:
a. We turned Alchemy into an all DC vessel. All we have to do is keep the batteries charged and all else happens automatically without supervision. This made sense to us as for vessel that was off the grid for long periods, but also has worked marvellously for a vessel wandering to different shore power grids, US voltage and Hz and now Europe voltage & Hz. All we need is a battery charger that works on different voltages and Hz (is easily attainable).
b. When we need AC power, mostly charging laptops, phones etc, we use a variety of inverters.
c. To sustain this we have a DC genset, basically a Kubota tractor engine attached to a 150 amp (12 volt) large frame alternator with a 3 stage programmable regulator. (When bought, our boat had an AC genset. We found it to not be sensible if all functions could be done by DC. Again, the only caveat is if you feel it necessary to have aircon at anchor. Then an AC genset makes more sense.)
i. Our choice of a DC boat and genset has made life aboard much easier. The genset has rarely not worked, but has been a real headache at times. One would think a small diesel turning a big alternator would not be a design challenge, but I have done a fair amount of research and no one has accomplished it yet.
d. We have not gone to passive systems as no reasonable passive system would meet out power needs exclusively. Therefore we need a genset and, if you are going to have a genset, we choose to run it an extra 10 minutes rather than have another system (or systems if a wind generator also).
A note to those with gensets like Colin’s that work on gasoline/petrol. At least a couple of US insurance companies (please check out present day policies as my data is 15-20 years old, but ins co’s seem to be getting pickier) would refuse fire coverage if any engine using gas was stored inside the boat. This came up for boats that stored their outboards in a locker and it did not matter if the outboard was run dry before storage. I assume the same would apply for the gas generators that are more common nowadays. A surveyor friend was required by one company to report outboards found inside boats for any claim.
My best to all, Dick Stevenson, s/v Alchemy

John Harries

Hi Dick,

Thanks so much for a great comment. We too have a generator and use it the same way you do—sparingly and efficiently. You wrote my comment for me, and, I think, explained it better than I would have!

Also, I am really interested in your point about the reliability, or lack thereof, of DC generators. Like you I have long thought that making AC to then convert 90% of it to DC, as most cruising boats do, makes exactly no sense at all. However, every single person I know that has a DC generator has had significant maintenance issues. I also know of a couple of real horror stories with these machines that required total replacement.

On the other hand, commercial quality diesel generators like our Northern Lights 5kw unit are fantastically reliable machines. Ours has run for 20 years without ever letting us down and in that time all I have fixed is one temperature sensor.

It’s a hard one: DC generators are much more efficient, but AC generators are much more reliable.

Having said all that. We are eagerly looking forward to the rest of Colin’s series. Given the advances in solar and wind power could we take out our generator and use the space for more batteries and then install solar and/or wind, and still not end up “camping”, as you put it so well? It would be a huge and expensive project. Would it be worth it? I don’t know. I suspect not. But if I were building a new boat, that is definitely the way I would look.

Colin Speedie

Hi Dick and John

Thanks for the wise and well made points pro gensets.

Lou and I like a very simple boat, and my experience of gensets has not been happy – but I’d agree that as things stand, if you want aircon or a freezer, a genset is probably the only sensible way to go.

But – there are promising new technologies that may one day make that achievable, and we’ll look at those, and also the way that improved existing technologies might make it possible to dramatically reduce the time spent running a genset (so saving fuel, and arguably making it live longer) – and who wouldn’t want that?

Look forward to your further comments.


vince bossley

Hi Colin, The old KISS principle is very relevant here and I am most interested waiting to read your first post on your new towed hydrometer – you don’t mention a brand yet. I had a Ferris and it performed remakably well, continuosly pumping in 11 – 12 amps at 7-8 knots.
When passagemaking it was retrieved at nights, but that still kept the batteries charged enough to run the watermaker and anything else required till next day when it would be deployed once again.
On passage in the Pacific, power requirements were absolutely minimal overnight and sometimes I even ran with the toplight off to absorb the wonder of the southern night sky.
Along with an Aquagen wind generator (not a lot of use sailing the trades) and a high output alternator on the engine, plus a small petrol Honda as a back up (but never used), we had surplus power at all times.
As an alternative to a prop generator I chose to fit a feathering prop and gain the extra knot or so in sailing speed, not to mention much less wear on the shaft, couplings, etc.
Towing a generator no doubt takes a little off your sailing speed, but the additional speed gained from a feathering prop more than makes up for that and I certainly could not detect any drop off.
The main downside was the tendency for the unit to occasionally jump out of the face of a wave, tangle itself and create quite a rumbling noise in the boat, ( not appreciated by the off watch sleepers at night!). At this point, the boat was stopped, the unit retrieved and re-deployed.
With a bit of practice, we could get this operation down to fifteen minutes and be underway again.
On two occasions in the Pacific, on retrieval, there were gashes on the steel shaft, so I can attest that large denizens may/do take a liking to the flashing prop from time to time. No doubt if the attacker had struck forward of the shaft one could imagine that the line could have been severed and the prop unit lost. A stern fitted unit would circumvent this potential problem.
The only time I ran the main engine was for motoring in a calm and entering and leaving port/anchorages.
I like the look of the new Watts and Sea unit and if it lives up to the published figures it should prove to be a suitable alternative. Not cheap, but no more expensive than a genset, and it is silent, instead of burning fuel gives you power, will not annoy anyone else and with much more straightforward maintenance.
Your boat moving through the water is the largest power generating unit that you have, so why not use it? Up until now, a towed or transom fitted hydrogenerator is the best that is available that will give you all the power and more that you want -delicious!

vince bossley

Colin, I would hasten to make a couple of corrections in my earlier post:
para 1: should read hydrogenerator not hydrometer
para 6: last sentence, ‘stern fittted unit’ should read as an immersible leg type as per the Watts and Sea units.
Hit the ‘submit’ button too quickly – my apologies

Ian Robertson

Have owned a cruising boat for summer use for 1 yr. Definitely on a steep learning curve, so very grateful for information.

John Harries

Hi Ian,

Thanks for the kind words.


Sorry, but I just had to post this video here:
Not a bad word about Hallberg Rassy. But you would need a nuclear powerplant to run this one.

Ee Kiat Goh

Hi Colin, on alternative energy, a marine electrician told me that if you have more than one charging source, there can only be one charging source at any one time (assuming you have only 1 battery bank). If that is the case, on a sunny and windy day, one can only have either the wind or solar charging but not both. Is that true?

John Harries

Hi Ee Kiat,

That is not correct , and shows that the person you spoke with has almost zero understanding of basic electrical principles—sadly a very common situation situation among “marine electricians”.

These basics are quite simple and easy to understand. I have explained it here: