Balancing Battery Bank and Solar Array Size

Eric's CS 36T sporting her solar panel.

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?

  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. Do You Need A Generator?
  22. Efficient Generator-Based Electrical Systems For Yachts
  23. Battery Bank Size and Generator Run Time, A Case Study
  24. Battery Options, Part 1—Lithium
  25. Battery Options, Part 2—Lead Acid
  26. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Load Dumps
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Ion Batteries Explained
  31. 11 Steps To Better Lead Acid Battery Life
  32. How Hard Can We Charge Our Lead-Acid Batteries?
  33. How Lead Acid Batteries Get Wrecked and What To Do About It
  34. Equalizing Batteries, The Reality
  35. Renewable Power
  36. Wind Generators
  37. Solar Power
  38. Hydro Power
  39. Watt & Sea Hydro Generator Review
  40. Battery Monitors, Part 1—Which Type Is Right For You?
  41. Battery Monitors, Part 2—Recommended Unit
  42. Battery Monitors, Part 3—Calibration and Use
  43. Battery Containment—Part 1
  44. Q&A—Are Battery Desulphators a Good Idea?

Eric started sailing and maintaining boats from a young age landing his first professional sailing job at the age of 16 and his first master’s license at 19. His sailing experience is highly varied, including multi-week open boat trips, working aboard tall ships, coastal and offshore cruising, and working in a shipyard specializing in large wooden vessels. Eric currently works as a mechanical engineer, designing products such as air compressors, steam expanders, and robotic arms. He enjoys sailing with his wife and two kids in their free time aboard their CS36T in the northeast US and Canadian Maritimes.

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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.

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

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.

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