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?
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.
Hi Paul,
Yes, 2 engines is definitely interesting as you can potentially improve your charging regime with more granularity of output. To me the biggest thing about cats is many have a built in place that is reasonable for solar although I have still seen plenty of examples that I don’t like.
Eric
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.
I’m glad to hear it was helpful. Eric
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.
Hi Daniel,
Thanks for the link to the Victron calculator. I had not previously seen it but it seems quite promising. There actually used to be companies who would go out and measure over a long period of time for big solar farms but I understand they mostly don’t exist anymore (a friend owned one).
I absolutely agree that estimating solar is not trivial. My first exposure was before there was great info available like there is now and I had to pull sun angle data, cloudiness data, snow coverage (this was for my parent’s house), shading and a few other factors together from different sources and put it together into some Matlab code. At least it was a grid tied system and I didn’t have to worry about extremes.
On the 0.77 conversion factor you use, I hope that the conversion from panel to electricity is better than that these days, most solar controllers (DC) or inverters (AC) are quite efficient. Are you including something like shading or something in there? Getting the energy into batteries, especially lead acid that are in absorption can be at a much lower efficiency but with solar, you don’t hit absorption until quite high SOC.
Regardless, trying to put a really fine point on this gets tricky because in truth, there will always be that horrible weather period or really high load period that is much worse than normal and you really should not design to that other than having an alternator or generator available to deal with it.
Eric
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
For Europe, EU provides a pretty nice tool for estimating solar yields
https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#PVP
Hi Henri,
Thank you for sharing that, it seems like a good tool. Playing around with it for a few minutes, I like that it has an off-grid calculator but I didn’t find a way to allow other charge inputs. Dealing with shading seems a bit tricky for the DIY’er, this seems geared for large solar fields. For the person who understands that but doesn’t have stuff like the irradiance data, this would definitely be useful.
Eric
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.
Very true, we all need to keep watching for those things that we don’t know we don’t know. Eric
As Nobel Laureate Howard Zinn answered when asked by students as to what advice he had for them, he responded: “Respect the unknown”.
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. 🙂
Hi Stein,
I am glad to see that your flow chart lets you get immediately to the go sailing level.
Cats are definitely interesting and can be easier to get solar on without as many negative consequences. Not only are you going onto an existing surface, they don’t heel considerably and that is a lot of the problem with big arrays on tall stern arches with monos.
Eric
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?
Hi Pepijn,
For the most part that is a batteries question and not related to the solar side. There is a small relation with lead acids as charge efficiency changes with state of charge and C level of charging so some setups will get a bit more energy out of their solar but it is a pretty modest difference.
To your question about cycle life, cycling less low in a lead acid battery bank does make it last longer. But I think you have to ask the question of what is your goal in doing that? If you are going to remote places for 5 years and won’t have a chance to swap batteries, you might want to do that. But if you just want to have a good performing boat and not spend too much money, I think cycling to 50% is a pretty good bet. For a typical GC2 golf cart battery, if we go to 50% SOC, we get 1200 cycles whereas if we go to 70% SOC, we get about 2000 cycles. If you do the math on that, you find that the total energy contained in those cycles is nearly identical so you haven’t magically made a given battery charge and discharge more energy over its lifetime, it is just that each cycle has less energy so it is over a longer time at the expense of needing more batteries aboard at any given time. This is obviously lab cycles and I kind of suspect that the bank being cycled to 50% will be better for most users in the long-run.
Eric
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??
Hi Michael,
Unfortunately I don’t think you are wrong, this is a tough problem. I have seen too many pictures of arches stripped off the stern of boats due to rough conditions offshore but we also need to keep the lights on.
To some degree, I see it as a matter of scale. A single small panel could be supported by a well-built arch but it obviously won’t solve your energy needs completely and will have performance impacts on the boat. Potentially this arch could be used in a coastal cruising mode and have the panels stored below (tall order for a small boat I realize) during passages.
Where I hope we are going in the future is to panels that can be walked on and are cheap enough and low enough carbon footprint that we can cover a lot of the deck with them and accept that there will be a lot of shading issues by simply having a much higher nominal output. I noticed that several IMOCA’s now have panels in places they are regularly walked on and an interview with a crewmember of Malizia specifically mentioned how easy it is to walk on them. So maybe this is not that much further off but I do still have plenty of questions in my mind.
I do think that a hard dodger with panels on top is good for a lot of people but output is limited, especially with certain sun angles. A hard dodger combined with some additional panels in other places might get you to where you need to be though. So all of this speaks to needing to focus on conservation with things like a windvane instead of an autopilot, if you passage only occasionally would you consider doing it with canned instead of refrigerated foods and then turn the fridge back on when coastal, etc. Also, very targeted engine charging can be a big benefit and if you do it every few days on an occasional passage, it probably isn’t too harmful to the engine provided you treat it correctly otherwise.
Eric
Hi Michael,
I realized that in my previous comment I assumed that you had already done what is being suggested in the article. If it isn’t clear, I do not recommend counting on significant solar generation during poor weather periods. Instead, you need some combination of enough battery capacity and alternative generation such as the engine, a hydro generator for fast boats sailing where it is windy when cloudy, etc. For example, your trips in your overlander may have worked just as well and your DOD might have also never gone below 40% with a single panel and a bigger battery. If you strictly follow conservation and then have a large enough battery (overly large doesn’t do a lot of good and has issues with size and weight), the actual wattage of solar required is much less than people often think but it is still enough to be very hard to fit in a seamanlike way if you have any big energy consumers like refrigeration, autopilot, watermaker, etc.
Eric
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 ….
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/
Hi John OK thanks for that, will have a read
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 …
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/
Thanks John will have a look..
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..
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.
Hi Jonathan,
Yes, the same analysis will be done for the A40.
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
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.
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.
Hi Pedro,
Yes, there are some good rules of thumb out there but the trouble can be applying them to your situation. For example, pulling 80A @12V off of a 15HP diesel which some 30’ers have probably isn’t a good idea whereas if you have a 75HP engine, you could pull double that. So I think it is best to look at this as a system but that can be tricky for people without significant training. I think you are wise to pay for the expertise if you don’t have it and won’t be able to acquire it for whatever reason. I hope they give you a detailed schematic for any future repairs or modifications and take the time to walk you through it.
And I am with Matt on solar development, there is no shortage of people working on it although most are working on it for other applications with more upside potential. But I do hope that we can get to some level of integrated solar as that will have huge benefits.
Eric
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
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.
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”
Hi Pedro,
That makes sense. I always find it funny how myself and others will say you should pay for the expertise if you don’t have it but then turn around and complain about the state of most boating service organizations. I don’t have a good answer to this other than the obvious checking of references and reputation, getting a very clear statement of work and checking in regularly. I don’t have a lot of experience with this as we do almost all of our own work but I have heard plenty of horror stories. In the case of an electrical system, people can optimize around very different things and you need to make sure they are optimizing to your preferences. For example, you can optimize around component cost, overall install cost, total lifetime cost, reliability, simplicity, fanciness, etc. My observation is that many people leave this up to the service organization who fails to ask and then does their best to guess the preferences and is often completely wrong.
Eric
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.
Hi Pedro,
No, I have never done the research and have no first hand experience with hiring anyone to do this so can’t recommend anyone unfortunately. Around here, it seems like the people who get good service they are happy with generally speak of Ocean Planet Energy which is Bruce Schwab’s (quoted in the article) company but I have never used them for consulting, only to buy through.
Eric
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.
Hi William,
Yes, I think that most boats at anchor can have significantly more solar than underway if people are willing to put the effort in. A lot of long term cruisers seem to stay in one place for long enough to have this make sense. In some places, I can see a boom tent making sense and in others, having something more on deck can make sense. I do get a bit nervous about settling the boat in too much as most of the places I sail suffer from some sort of squalls and dragging anchor while trying to climb around undoing everything doesn’t sounds like the greatest idea, I have been there and done that on some much larger vessels. I think you are right that flexible makes a lot of sense here due to storage but that does mean effort needs to be taken to limit flexing both in terms of magnitude and frequency as that can quickly wear out panels.
Eric
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
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
Hi Brian,
What you are seeing with getting to high SOC charge very quickly is a function of the compressed time scale on the X axis of the graph. If you look close, the SOC line is not quite vertical in those areas but very close. It might be a bit easier to see in the alternator output curve where you can see that it is not just a vertical line but 2 lines slightly spread out down at the bottom of the graph showing it happens over a few hours. I chose to show 2 weeks as it was a good balance of showing what the article was attempting to show but the major downside of it is that you can’t actually see these little intricacies. When modeling, I like to put in lots of little automatic checks as well as doing my own sanity checks as it is very easy to put in an error and this model has automatic checks for going from 80-90% SOC and from 80-100% SOC specifically for this reason. Most factors that I know of that can play into this are included but there are a few things like temperature affects that are ignored as they are small and tricky to model well.
I appreciate the check as one of my fears in this stuff is always to put something out with a strong conclusion only to find the model had an error in it.
Eric
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
Hi Jan,
Thanks for the real world truth test on that, always the most useful information.
(tried to post unseaworthy photo of 2kw array, but it appears attached pics dont come through)
Hi Conor,
See comment guidelines for how to post a photo.
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.
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.
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.
Hi Rob,
I like that you remove all that top hamper of the Bimini and panels when at sea, very sensible.
Hi Paul,
You are absolutely right about this, and I think your terminology of managing the supply side is good. I tried to give it a small nod with my mention of behavior modification at the end of the article but it is its own subject that people can get a lot of benefit out of. The modification level you are describing is quite reasonable and I would hope most people would do it but if you want to take it further, then that can become more problematic as it can cut into creature comforts. Like you, we try to be aware of when we have excess charging capability and take advantage of it.
I occasionally toy with the idea of an electric outboard and I have modeled it some. If we just plugged it in and charged it whenever we got back to the boat, our current system would be totally inadequate but if we were to be smart about when we charge it, it appears to me that we could get by with minimal to no changes to our current system.
Eric
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.
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.
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)
Hi George,
I have not seen actual data on this and I would think it would depend a lot on heel angle of the boat as that will change how the air flows across the blades.
If you want to get a rough idea of the drag, figure out the power output for the apparent windspeed you will see and then divide it by the blade efficiency (note this is not the efficiency usually cited which is the ability to capture all the energy of the airstream, this is a measure of how much usable power you get out versus power taken out of the airstream). This is not data I have ever seen on a small boat turbine so I would guess it is around 50% at best.
The bigger issue that I often see with wind generators is how they are mounted in relation to solar. I continually see boats with an arch full of solar panels and then a wind generator mounted directly over 1 edge of the solar array so that it provides a lot of shading. For many of these boats staying out of constant high wind areas like the trade-winds, I suspect that they would generate more power simply by removing the wind turbine and focusing on limiting shading on the solar.
Eric
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.
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.
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.
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.
Hi Peter,
Here is where that number came from. The International Energy Agency (IEA) cite the number 2560 kg CO2e/kWp. When I dug into the source of that number, it appears to be a paper by Ito in 2011 which is available free here: https://www.intechopen.com/chapters/17733. Finding that source definitely makes it clear that this number is somewhat old and there are some sources claiming that is has gotten much better since (like 3X maybe) but in a few minutes looking tonight I did not see one from a source like IEA or EIA.
The diesel number comes from EIA (https://www.eia.gov/environment/emissions/co2_vol_mass.php) and is 10.19 kg CO2/gallon and I think this is a pretty widely used number and basically what you give.
Obviously 136 and 2560 are wildly different numbers and I can’t fully explain the difference. I suspect Maxeon has kept their scopes narrow whereas the IEA numbers try to really look lifecycle for the panels. These reports focus on utility scale solar so they don’t apply perfectly but in general I think they are indicative. Also, Maxeon is using the panel ratings whereas the IEA number is using kWp which takes into account things like installation angle so will take a hit there too. I certainly hope that the number is lower than the IEA number and that it does continue to go down, that can only benefit everyone. I would also be interested if anyone has a gold standard type source of the current state of lifecycle emissions from solar and with enough information to calculate it back to kg CO2e/ kWp as a few sources report /kWh which isn’t applicable to off-grid cruisers.
Eric
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.
Hi Jim,
I am glad you found it helpful.
Our biggest load is the refrigeration which is a relatively constant load in effect as the cycling happens much more frequently than a charge cycle. We do not have a watermaker but they are an example of a large energy demand but also one that you can have some choice as to when you run it. The days with big SOC drops are due to low solar output as you point out but also due to days where there was little or no motoring. What matters is what your daily energy usage is and whether it is pretty constant or varies wildly (ours is pretty constant).
I think that we will keep the same size battery bank when the time comes to replace it but it is not totally clear cut. The example above was a single trip and it happened to be one with decent solar output, a little more motoring than normal and was before we redid our navigation electronics which has raised our daily usage a little so is a little on the optimistic side. I believe that we would be happy if we dropped our bank size by ¼-1/3 with our current needs. Unfortunately, using 4 GC2 batteries, our only option is to halve the bank size which will cause us to fall off that battery size cliff. However, because we are not full time cruisers, it wouldn’t be totally unreasonable to plan to mitigate that cliff with a bit more engine charging as much as that annoys me.
In our case, we decided to put in the effort into mitigating having 4 GC2’s by moving the battery bank to be low and central in the boat, when we got the boat it was in the stern. I have a friend who struggled for years with lots of engine hours to charge and short battery lives who happens to have loads nearly identical to us. On his boat, the batteries are literally mounted to the transom and there was not a great way to move them so we elected to do 2 GC2’s, 160W solar, be willing to go a little lower on SOC and do a little more engine charging. It is all trade-offs and 2 extra batteries in the right place is not the end of the world but if they were located poorly, I would definitely work really hard to have a smaller bank.
Eric
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/
Thanks for the replies guys, big help as always!