The Offshore Voyaging Reference Site

Balancing Battery Bank and Solar Array Size

John here: Phyllis and I are excited to publish Eric Klem’s first article for AAC. That said, Eric is far from new here. For years, in the comments, along with Matt Marsh, he has been helping us all understand the engineering that does, or at least should, drive every decision we make about how to equip and operate our boats. Over to Eric:

One of my favourite gear additions to our cruising has been a solar panel. Prior to adding solar, our batteries did not last long and we regularly had to ruin the peace of quiet anchorages with hours of engine charging.  

At the same time, I know that the addition of solar has negatively impacted our boat’s performance, and I’m conscious of the need to avoid overdoing it.  

Often, sailors focus on the question: 

What mix of energy storage (batteries) and energy generation (alternator, solar, etc.) will meet the energy needs of my boat?

But if we simply solved that problem, we would be ignoring the higher-level problem statement:

How do you optimize the boat as a system to make it comfortable while maintaining performance and seaworthiness?

The major trade-off here is sailing performance against electrical performance.

Of course it’s impossible to define the “right” trade-off for every boat and sailor. Racers might be willing to replace their batteries every season in exchange for lower weight and no windage from solar panels or wind generators, while others may be willing to give up performance in favour of increased comfort.

This article examines how to optimize this trade-off for people who do not want to run their engine or generator exclusively for charging and are willing to make the trade-offs to do so.  

For people who run a boat with large electrical demands that necessitate a generator, John already has an article on optimizing that—see Further Reading.

Why Not To Oversize

Because everything on a boat is a trade-off, there are negatives to oversizing a system that can impact boat performance, your enjoyment of your boat, and perhaps even your safety.

Windage

For example, say that you are sailing to weather in your 40’ cruiser at 6.5 knots on lovely flat water with a 20-knot breeze and 20° heel.  

The total force propelling your boat forward is going to be surprisingly low at around 1000N (225lbf).  

Then let’s say you add a 1kW solar array on a tall arch. In the relatively undisturbed air back aft, that solar array is going to add around 200N (45lbf) of drag, being about 150N from the panels themselves and the rest from the supporting structure.  

This 20% loss in net propulsive force is enough to slow your boat down from 6.5 to 6 knots, ouch—likely a bigger hit to performance than downgrading from a feathering propeller to a fixed one.  

Weight

Windage isn’t the only thing to watch; weight is also important.  

Batteries are not too bad if installed low and near the centre of the boat—only racers will be likely to tell the difference of increasing the bank say 200 amp hours (Ah) at 12 volts here.  

But solar panels are a different story, as they are usually placed higher where they decrease the righting moment, and far aft where they will also increase the fore and aft moment of inertia, thereby causing the boat to pitch significantly more.  

A 1kW array of good, rigid panels will weigh around 50kg, and the mounting structure will likely be at least as much again, so call it 100kg (220lbs) in a bad place.

Take It Offshore

Let’s go back to that lovely upwind sail we were doing but now put it offshore. 

20-knot winds mean something on the order of 8-foot (2.4m) waves with a reasonably developed sea state.  

Between that extra weight and windage on the stern, I would guess that we might find ourselves dropping from 5.5 knots average to 4.5 knots or worse, and averaging a few degrees lower on our course made good.  

I suspect most AAC members have been in the situation where the boat is slow and wallowing, which is rapidly fixed by just untucking a reef, showing how sensitive boats can be to drive force. But we can’t so easily change the weight and windage of a solar array.

So on a passage with a lot of windward work, this 1kW array might make our passage 25% longer, and the results will look even worse as the wind increases further.

Another noteworthy consideration is sight lines; solar panels often obstruct your view of the sails and/or the ability to see other vessels.

Finally, there are the safety aspects if a wave sweeps the boat. John has covered this in the past (Further Reading) so I won’t cover it again here.

A Balancing Act

So the key questions that we must balance are:

  • Is this solar array:
    • hurting performance too much?
    • adversely affecting our ability to sail the boat safely. 
  • Is it big enough to keep up with the electrical loads?

Other Considerations

Conservation First

Optimization should always start and end with conservation. Powering refrigeration, autopilots, watermakers, computers and other large loads is very challenging and minimizing these loads is critical.

Efficiency

Related to conservation is making sure that components are as efficient as possible. On the demand side, this means doing things like re-insulating your icebox and investing in LED lights.

Looking at efficiency in electrical generation:

  • Having a powerful alternator that is properly regulated is key, I like ~3A/engine hp.  
  • With solar, I look for a minimum of 22% and prefer 24% efficient panels.
  • Install a quality MPPT controller.

Arriving At The Right Balance

So now we have the problem defined, how do we solve it in the best and most balanced way?


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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. Lithium Buyer’s Guide—Budget: High End System
  35. 11 Steps To Better Lead Acid Battery Life
  36. How Hard Can We Charge Our Lead-Acid Batteries?
  37. How Lead Acid Batteries Get Wrecked and What To Do About It
  38. Equalizing Batteries, The Reality
  39. Renewable Power
  40. Wind Generators
  41. Solar Power
  42. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  43. Battery Monitors, Part 1—Which Type Is Right For You?
  44. Battery Monitors, Part 2—Recommended Unit
  45. Battery Monitors, Part 3—Calibration and Use
  46. Battery Containment—Part 1
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Paul Rutherford

Thanks for your article Eric, I’m liking the more holistic approach rather than just loading more batteries and panels onto the boat. Of course if you have 2 engines (catamaran) you can add more alternator input and or charge with one engine only if a top up is needed.

John Michaels

Thank you. I was about to step in and make every single mistake you mentioned!!
Instead, I may save a few thousand dollars and still sail and live efficiently on my sailboat.

Daniel McCarty

Estimating solar panel production is not easy.

Years ago, when looking at solar power production on the house, the rule of thumb for my area was we would get a little over five hours of solar production during the summer and a bit less than five hours in the winter. Looking at various calculators over the years, the number of production hours has changed, though the year round average is still five hours, the number of hours is quite a bit lower in winter and higher in summer, in the more recent tools.

Another rule of thumb was that the amount of power available for use would be .77 of the power produced by the panels. I don’t think that number included battery storage so OPE’s .6 factor seems to be pretty good.

Victron has an interesting calculator, https://www.victronenergy.com/mppt-calculator that uses location like NREL. I wonder if the Victron calculator uses actual number of sun hours vs a number generated by latitude. The Vicron calculator has different results for European cities that are pretty close in latitude which makes me wonder if it is using actual measured data showing the effects of weather. Anyway it is another tool to compare data.

Daniel McCarty

Eric,

Sorry for the late response but work and family has been consuming too much time. 🙂

The .77 number was from a solar power class I took 5-10 years ago for houses, not boats, so shading from structures or trees should not have been an issue. However, they might have included cloud shading. The factor was the energy loss from the panels to the power available at the outlets. I don’t think the factor was used in regards to battery usage, but just to determine the number of solar panels wattage one needed.

There was an online calculator, which I can’t find, that used the .77 factor. I did find this link, https://www.calculatorpro.com/calculator/solar-power-calculator/, where they use .8 as a factor, so close enough. No mention of batteries or shading.

Dan

Henri Bergius

For Europe, EU provides a pretty nice tool for estimating solar yields
https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#PVP

Alan Bradley

Thanks Eric, for an eye-opening and very informative article. Once again, it’s the things you don’t know that you don’t know that can get you into trouble.

John Michaels

As Nobel Laureate Howard Zinn answered when asked by students as to what advice he had for them, he responded: “Respect the unknown”.

Stein Varjord

Hi Eric,

Thanks for an easy to follow explanation of a topic that doesn’t always seem easy. I’m already a bit of a nerd on this general topic, but my understanding has been improved.

I must admit what I’ve done with solar is to place it on the (catamaran) cabin roof, where I get no extra windage or other issues and can easily have far more than 1 kW nominal. (I have about 600 W now). That means I never need any other charging. My system isn’t balanced, but at no disadvantage. I still have made sure our consumption is minimal.

For fun I once made a flow chart to a friend. I think it was about as follows. Feel free to use it as you please.

1. Does what you have now work well enough?
Yes: 7 No: 2

2. Are the present batteries OK healthy?
Yes: 3 No: 6

3. Can you fit solar panels nicely on the boat without an oil rig etc aft?
Yes: 6 No: 5

4. Is your present alternator system reliable?
Yes: 1 No: 5

5. You have to figure this out, or pay somebody to do it.
Then: 1

6. Just do it! (No duct tape or WD40).
Then: 1

7. Go sailing. 🙂

Pepijn Toornstra

Hi Eric,
Very insightfull. Thanks.

Regarding increasing battery capacity resulting in lower DoD. Is it not that a lower DoD will disproportionally increases life-cycles, thus improving battery life?

Michael Van Eeden

ja Im stuck. I have had solar on my overlander truck and traveled for 7 years in Africa.& USA

2 x 150 w panels flat, on the roof rack and 200ah agm with vectron 30amp mppt worked great ,

Running fridge freezer in 38c+
charging cameras drone laptop phone internet booster router and so on. Never went below 50% more like 40, Lots of sun in Africa year round.

But now I’m back on the water,where I began life sailing from 6y. I just bought a 34foot sailboat and it has a very well built arch tuff lots of money spent. But….I want to sail from South Africa to Australia and around to Caribbean..

But I must say, Solar Arch’s davits scare me in every-way for offshore, makes no sens at all, for seaworthiness, changes all the geometry of the boat.

if I get knocked down, will I come back up? Will the panels still be there? Some of the arches I see people put on, flimsy and huge 600 800w, crazy. its fine if you stay tide to the dock or C cruising.

And all the other reasons, Like dont have things sticking on the ass twisting the boat and pitching it, stressing my hull.
So the arch is deff coming off my boat.
People think I’m nuts for taking it off.

But now what?

Diesels do not like to idling at all,well not propulsion ones. Piston glazing not good. Shortens life big time. Not a good option while not moving.at least 1800 rpm.and who wants to sit on the chain at 1800rpm or more..Its a good way to clear out a tight anchorage for yourself LOL

So I”m stuck,

The only way I can figure, is build a hard dagger
witch I plan to anyway, and putting soft-panels 365w, only need 200w but soft panels are not as efficient as hard panels, as you all prob know.

And well as much as I hate the noise and disturbing people around me,

I’m going to have to still burn Fossil fuel for a wile longer and by a Honda geni to top up., better than un-safe sailing and the sea
will just knock it off the arch and panels anyway, In a big blow and all that money gone and might put big wholes in your boat while out there?

So I think, what’s the point in stressing my boat and me with this big arch on the stern..It Don’t seem natural being there anyway.

If I had a 50-60 footer, then maybe a strong low-key arch would be fine built into a dagger frame type…But on a 34 to 40 foot its just not worth the safety downgrade for me.

We spend all this time and money getting a great quality off-shore sailing Yacht and then destroy the balance…IMOP

So its a really tuff one for me , how do you stay safe and run all your needs, and be green and protect your engines life span ??

It must be soft panels, everywhere you can, and keep stuff off the transom….I dont know, do I have it all wrong??

Michael Van Eeden

Yes I here you Eric, I want the walk-on panels, thats the answer for me. But until then,

its going to be soft panels on the doge and small Honda geni backup, Also a backup bigger alt on auxiliary power.

Might as well take full advantage when paying to burn diesel when its cloudy or doldrums But Thanks More thinking needed ….

John Harries

Hi Michael,

Sorry to add another complication, although I have done it, there are big time safety issues with using a Honda through shore power: https://www.morganscloud.com/2021/09/07/why-i-wont-power-our-boat-with-a-portable-generator/

Michael Van Eeden

Hi John OK thanks for that, will have a read

Michael Van Eeden

OK John I read that, scary, But I was thinking of using the Honda to charge the battery’s with a smart charger?

No plunging into mains in fact my boat will only be set up to run 12v as I’m always out on the hook exploring somewhere, so I dont need a 240v/120v system at all on board.I will have a 3000w inverter clean one..

So I can not plug the portable geni into ships system anyway.

Used as ac/dc changer, or is it still dangerous?

I get that Im going backwards to the stone age of fuel burning, but I will have to have fuel for my tender anyway.

And here in south Africa we pay 16 to 26 x to the dollar, so not all of us have money for nice trick modern solutions like the rest.

Everything we buy is from USA Or UK. Just a life raft is R33.000 to R50.000. Anchor 25.000 to 30.000
Mast / rigging R150.000 to R200.000 sails 100.000 weather fax 50.000 VHF icom 32.000
GPS / Plotter 30 to 60.000.

1x 102amp AGM 3500.00 x 4 Vectron Multy plus would be great but its 38.000 , no can do..

In South Africa it cost R400.000 to half mill to refit older boat and have it safe to go to sea..

So you see I need to find simpler cheaper solutions to put my boat together and keep it running.

And thats why I was thinking,

250/300 amp AGM with 200w panels on a hard dogers roof and 120amp alternator on my Bulk DV20 when on the move and then a portable backup to charge batters. from a smart charger as last option.

I cant use my main engine to charge the battery’s every day its just not good for it, to ideal that long that much..

So do you thing its still a bad idea to use portable geni to charge battery’s only ? small inverter portable genis are cheep here because we have so much load shedding (power black outs every day),

Most days its 8hr a day spread out in 2hr and 4hr intervals in 24hrs. so clever affordable ways to set up a boat is a must…

Thanks John, Im just learning here so bare with me …

John Harries

Hi Michael,

I guess that would be OK. That said, as I say in the article there are a lot of variables in the way a generator is wired, and I’m not an expert on that, so I can’t be sure.

Here’s another possible option, if you can get one inexpensively enough in SA: https://www.morganscloud.com/jhhtips/portable-solar-panels-for-cruisers/

Michael Van Eeden

Thanks John will have a look..

Michael Van Eeden

I must say, there is so much great usable intel here, Thanks for that John,your doing a great service here…When you go out onto the web to get answers you soon realize the value here…Thanks For That everybody..

Jonathan Cohn

I’m looking forward to this kind of analysis for the Adventure 40 focusing on its offshore sailing mission. I know energy draw will vary by user but I hope some leeway is allowed for users adding instruments and refrigeration.

John Harries

Hi Jonathan,

Yes, the same analysis will be done for the A40.

Pedro Fernando

nice article
ive came across some info, which is perhaps by now outdated, stating some key point like:
size your battery bank up to 4 times your charging ability; invest in a 120 amps Balmar but run it at 80 amps.
thar sort of stuff

i guess we are waiting for “someone” to get serious about solar and incorporate tech that its already fact, such as paint embedded solar cells, so you can gelcoat/paint thad deck and have 2 wires coming out of it straight to your chargers, or integrating cells into the sail cloth

Matt Marsh

There are a lot of “someones” who are serious about pushing new solar tech.
However, there is a vast and often unbridgeable gap between “it works in the lab” and “it can be made, sold, and supported economically at scale as a commercial product.” In tech-startup land we call this the Valley Of Death, and a significant majority of new technologies that work in the lab and get fawning “OMG this will revolutionize everything!!!1!” media coverage end up vanishing into it as the true complexities and production costs become known.
If you are actually trying to build a system that works, affordably, with things you can buy commercially, then the approach Eric described in this article is really quite good.

Pedro Fernando

hi Matt

I am actually trying to build a system.
thats why im paying Pacific Yacht Systems to design it, spec it and then i will build it resorting to their diagrams.

There is just too much info in this ever growing field to keep up with. Not impossible if you can dedicate all of resources to it.)or at least a good chunk)

Presently, i cant, thus i pay others.
It aint cheap, but at least i know its well done and i can learn alot from it by “reverse thinking” on what they spec, based on their decisions.

Pedro Fernando

Hi Eric
thanks for your reply

the reason is time and a lack of a (enough for me) backgroung to allow for a shorter learning curve. i can deal with everything else on the boat, there is nothing im unable to tackle but electrics its some other stuff.

it aint difficult to get the basics but the amount of gear being developed and their associated ideosyncracies makes it wise to just ask for help, specially when you are gearing your boat as a go-anywhere one.
Detailed diagrams will be provided since im the one doing the actual building-connecting-stuff-to-stuff.
I learn well by doing and i face it as a electric workshop (thats how im justifying all that money in my brain ahahah)

Best
Pedro

John Harries

Hi Pedro,

I think that’s a very smart way to look at it. If we try to be experts on everything on a modern boat we will either screw up a lot, or never get out sailing, and probably both.

Pedro Fernando

indeed, the refit still has 2 strong years ahead for completion,and eventually i would like to shake it down and improve on those systems, thru sailing, which will take more seasons to achieve as there is always something “improvable”

Pedro Fernando

i see your point
the first thing they gave me was an excell file for me to fill up with all the gear i will install and tweak their hourly usage to my needs (the power consumption numbers were also included). another excell was given to fill in about solar power array im expecting to get. so far they seem pretty professional, i came across that company on Ytube, the guy has lots of seminars online, and really appears to know its stuff. i think ive done a good decision. do you know more companies doing a good job on this subject?. thats valuable information for all of us and those ones should be rewarded by being publicized on places like AAC (not that im trying to tell AAC what they should do). all the info is out there, but having reliable hubs of said info its hard to come by.

William Koppe

Hi Eric,
I agree that fixed solar panels, ( unless on coach roofs) are unsafe.
Using flexible panels is much better, and they can be stored under bunks while on passage.
When at anchor they can be velcro attached to a boom tent wth horizontal supports, which can have the angle adjusted athwartships, as well as fore and aft, by adjusting the hold down lines and raising/lowering the boom.

George L

A friend of mine puts foldable panels on top of boom and sailcover when not sailing and that works really well. In an emergency under anchor, they could stay on. Even if they have to get off fast, it takes about as long as taking a sailcover off.

To me, until step-on panels are common-place, that’s the way to go. On most ships, every piece of deck surface is fair game to step on and any kind of precaution for fragile kit is out the window the moment conditions get worse

Brian Russell

This is an informative approach to evaluating one’s energy production requirements, thanks for the great effort!
Like in weather modeling it is the beginning assumptions that drive the process, so these need to be carefully established.

One question: In the graphs the SOC is shown to rise very rapidly during the engine charging time of less than 1 hour. How can one achieve 95%SOC in this short time period on lead batteries with their attendant charge tail-off in absorption phase?
Keep up the good work,
Brian on Helacious

Jan Schütze

Here at the north of Europe (54°N to 56°N) with a 100W portable solar panel, over the last two summers, it was a good day when I got 300Wh and above. So this number is for minimum shading and good alignment on a sunny day and it fits well Bruce rule of thumb. 300Wh is about 23Ah @ 13.0V.
Just for comparing a real world number against the calculation models.
Thank you
Jan

John Harries

Hi Jan,

Thanks for the real world truth test on that, always the most useful information.

Conor Smith

(tried to post unseaworthy photo of 2kw array, but it appears attached pics dont come through)

John Harries

Hi Conor,

See comment guidelines for how to post a photo.

Paul Clayton

I really liked this article, especially the holistic concepts of balancing supply and demand. As Eric put it early on, conservation comes first. It’s easiest to think about this as managing demand, but the flip side that doesn’t get as much thought is not wasting the supply. Practically every bank gets to the float stage sometime, whether it is on a bright, sunny day or while the boat is motoring in a calm or through a long canal, and at that point supply is being wasted, as the regulator or controller is limiting charging current to the battery. This is the time to charge all the devices – laptops, phones, flashlights, even in one case a CPAP machine – that friends and crew bring aboard. I believe in distributed power and have various puck-lights and fans running off double and triple A batteries about the boat, and a collection of batteries charging anytime we are in float. Reducing wasted supply makes it easier to live with moderate-sized alternators and solar arrays.

John Harries

Hi Paul,

You are absolutely right about using excess power intelligently. In fact I have written about exactly that: https://www.morganscloud.com/2019/01/09/cruising-boat-house-electrical-system-design-part-1-loads-and-conservation/

So doing was hugely effective for Phyllis and I when we lived aboard.

Rob Gill

Useful perspective, thanks Paul,

We have a “nice problem to have”, caused by our dual usage profiles. Our NZ summer solar supply is really overly generous, but well sized for SW Pacific Islands in the Southern Hemisphere winter (dry) season. But it means In NZ we are often fully charged on solar by midday.

What we have been doing is heating hot water in our cylinder, using the inverter. It’s not a perfect energy sink, as our cylinder draws about 65A when on, and our peak ever NZ summer energy supply from our solar was 53 Amps. 35 A is a fairly typical summer average for us and about 20A average for the Pacific Islands in winter (less hours though).

This means the batteries are making up the shortfall between the cylinder draw and our solar supply, so we only ever do this on fine sunny days in NZ. We prefer it to running the engine, or dragging solar showers around the deck (which we do in the Pacific Islands).

On a typical fine NZ summer day we have a fully heated cylinder and are back to 100% batteries by evening. Hadn’t thought about charging our devices as small energy sinks, but we will do so now…especially up in the Islands.

We have four 140W light weight flexible panels mounted on our bimini, and two smaller flexible panels on our hard dodger. Total 700W. The bimini is stowed in a cover for long offshore passages and the panels stored in the fore-cabin. We are very pleased with the outcome, as a bimini in NZ is almost essential equipment, with our very high incidence of skin cancer.

John Harries

Hi Rob,

I like that you remove all that top hamper of the Bimini and panels when at sea, very sensible.

John Harries

Hi Eric,

My modelling on an electric outboard for the J/109 yielded the same conclusion: not a hope on the batteries. So we went with a bit bigger alternator than I would have otherwise which should allow us to charge the outboard battery any time we motor for more than about an hour, which will generally happen at least once a week on any cruise here in Nova Scotia. And then of course we will charge the outboard any time we are on shorepower.

John Maturo

I did not do any calculations, but here is my setup that worked very well last summer on my solo cruise from Branford CT to Halifax and back. I sail a fully refit 1978 Baltic 39. I have 4, 6 volt golf cart wet cell batteries chained together into 2 sets of 12 volt batteries with a total of 460 amp hours in the house bank, a Balmar 80 amp alternator with controller, a starting battery on an echo charger, and 200 watts of flexible solar panels on my bimini with controller. I run digital radar, auto-pilot, 2 refrigerators, sailing instruments, 2 MDF’s, AIS, VHF, stereo with speakers below and at the helm, LED lighting courtesy and nav, and have a 1,000 watt pure sine inverter for charging tools, phone, laptop, and handheld vhf units. I never used my engine to charge last summer. I was surprised at the efficiency and light weight of the Flex panels. I sewed them to the bimini and will devise a velcro system this summer for easy removal. Being on the Bimini adds no additional windage burden and the weight is negligible. The unit is a Go Power kit from Defender with a great blue tooth enabled controller/battery state function.

George L

Excellent analysis, thanks.

Do you have any idea about the windage of a wind genator – let’s say 350 W, 1.2 m/4 ft rotor diameter? (at least there is some offset as it will perform better to windward than when running)

Peter Libre

The estimate that 1kW of solar has embedded carbon emissions equivalent to 250 gallons of diesel is high by an order of magnitude. SunPower panels use Maxeon cells; Maxeon reported in 2022 (page 25 of https://sunpower.maxeon.com/au/sites/default/files/2023-06/ESG_Report_2022_SunPower.pdf)
that its emissions were 136 tons of carbon equivalent ( which means emissions of greenhouse gases other than CO2 were included) per megawatt of production. Since a kW is 1/1000 of a MW, this equates to 0.136 tons/kW which would be 136 kilograms/kW. Since burning one gallon of diesel releases 10 kg of CO2, manufacturing 1 kW of solar embodies global warming emissions equivalent to about 14 gallons of diesel, not 250.

Stein Varjord

Hi Peter,
I can’t confirm or oppose the numbers, but they seem realistic. I’m not surprised by how absurd the earlier numbers are.

The problem with this type of quantification on this type of topic is that there are so many out there with an agenda and low competence or zero honesty. Lies repeated enough times seem to gather credibility, in some environments.

Flat Earthers are still a minority, but their way of reaching conclusions seems to be gradually more accepted.

John Harries

Hi Peter and Stein,

I personally have no opinion on this, or knowledge, but what I would say is that Eric is a very careful professional engineer who is never afraid of saying he does not know. Therefore, given that he provided a number I would be surprised if it were an order of magnitude out. Eric is also very realistic and not biased to either the fan boy or sceptical camp.

For example, in other threads you will find him defending electric cars when the matter of embodied carbon comes up.

Given that, I would put his opinion way ahead of numbers expressed in a report by a company making cells, hardly an unbiased source.

Peter Libre

The commercial price for utility scale solar PV is <$0.50/W. Even SunPower flexible panels can be purchased for $1.15/W in small quantities. If manufacturing 1000 watts required the equivalent of $750 hydrocarbon energy, the present market pricing would be impossible.

Jim Schulz

Eric this article has been the most thought provoking I’ve read with regard to battery bank sizing. With all the great products out there it’s easy to get sucked into a mindset of “how big a battery bank can I fit into my boat” rather than thinking of a holistic system of chosen loads, bank size, and PV array for the given use case.

The scale is also helpful because we have an Ericson 38, a boat somewhat similar in size and displacement to yours.

A practical question – after modeling, would you downsize your battery bank when time came to change batteries? If not, why not?

Also, you mention a water maker a couple times in the article. Did you run a water maker in the scenario and if so, when? I see big drops in SOC on certain days but they also coincide with low PV input days so it would be interesting to know.

Thanks again for this article, it’s helped a lot.

John Harries

Adding to Eric’s, excellent comment, we also have several chapters that cover the relationship between loads and sources that may help: https://www.morganscloud.com/2019/01/09/cruising-boat-house-electrical-system-design-part-1-loads-and-conservation/

Jim Schulz

Thanks for the replies guys, big help as always!