How Much Solar Power Do You Need?

By Carolyn Shearlock © 2015 • all rights reserved

How Much Solar Power Do You Need on a Boat? Lots of people tell you how to figure out how much solar power you need, but are they really accurate?

Lately I’ve seen several links on Facebook to various articles on determining how much solar power you need for your boat (or camper, or even a home).

After reading several, I felt compelled to chime in with my two cents. And, as is often the case, my views are a little different from the experts.

Basically, I think all the info about how to compute your electrical demand and calculate how many watts of solar will meet that demand are a great starting point. I just don’t think they are the ending point.

I think that neither calculation can be made with any degree of precision in general and the estimates are even more likely to be wrong if you haven’t yet cruised for more than a week or two at a time or only in a different locale than your long-term cruising plans.

The Demand Side

Typically, these articles tell you to do an energy audit. They say to list every single electrical appliance on your boat, figure out how much each runs per day, look up how much each draws when it’s on, and calculate the total usage in terms of amp-hours.

It’s great in theory.

But it’s virtually impossible to know in advance what you’ll use and for how many hours each day! It’s particularly hard to know before you’ve cruised in a particular area, but even experienced cruisers have a hard time knowing exactly what their usage is.

  • There can be considerable variation from one day to the next in general.
  • If you cruise to a warmer climate, the refrigerator and fans will run far more than you expect. There’s both the temperature difference and — for the refrigerator — you’ll be drinking far more cold drinks over the course of a day, and every warm drink that has to be chilled increases the load on the refrigerator. Seasonal variations will occur too! It’s not unusual for the refrigerator to run twice as much (or more) when full-time cruising in warm climates than it did on weekends in cooler climates.
  • If you have a 12v watermaker, you’ll probably make more water than you expect when it’s hot, both for drinking and for showers.
  • Friends may stop by to chat and you end up with more lights on and maybe some music too. Oops, that wasn’t in the energy budget.
  • How can you estimate an “average” amount for extra lights to work on an emergency repair?
  • Taking photos, posting to a blog or Facebook or even just checking on weather can add quite a bit as cameras, computers, tablets and cell phones all have to be charged.
  • And if you’re using an inverter to convert DC to AC power, you can check the AC draw of the appliance, but it’s hard to know just how much will be lost in the conversion. It’s often in the 10-20% range, again meaning that you’ll be using more DC power than you estimated (unless you included the conversion loss in your estimate).
  • If you are sailing (motor off) you’ll use more power than at anchor due to instruments, autopilot and radar in addition to all the “normal stuff.”

We’ve also found that what the manufacturer calls an “average draw” for an appliance that cycles (such as a refrigerator) often just doesn’t match what happens in real life. They may say it’ll run 20% of the time . . . but assume that the door isn’t opening, you’re not putting anything warm in, and the temperature is 70 degress. So they may say it has an average draw of 2 amps, or 48 amp-hours a day.

Then you’re getting into it a dozen or more times a day, putting a dozen warm drinks in, and the temperature outside is 90+. Suddenly it’s running 50% of the time and actually drawing 120 amp-hours a day. That’s a difference of 72 amp-hours between the numbers you used for your energy budget and what your actual need is (NOTE: these numbers are for illustration only and probably don’t reflect your refrigerator usage — in very hot weather we’ve had our refrigerator run 56 minutes out of every hour; in cool spells it may run only 10 minutes an hour.)

The Solar Output

How much a given wattage of solar panel will actually put out on a given day — or even on average — is equally uncertain. Was it cloudy? Sunny? Did the boat lie so that there were shadows on the panels for part of the day? How many hours of usable sun are there where you are? How clean are the panels? If you’re sailing, your sails are likely to shade one or more of your panels, too.

Again, you can make some general estimates but realize that they are only estimates. And yes, the brand/technology of the panel(s) and the charge controller will play a role in the actual production.

Bottom Line

The bottom line is that until you start cruising in your intended cruising grounds with your setup, it’s impossible to know if it’s adequate or not.

Take us, for example. We put a 345-watt high voltage solar panel with a Kid by Midnite Solar MPPT controller on the boat last spring. At the time, it was more than adequate for our needs even with a watermaker We were fully topped up by 11 AM or noon most days. But we had a propane refrigerator, which we just changed to a 12v model. So our demand has gone up.

But how much has it gone up? We don’t know exactly and it will vary from day to day.

And the question remains: Do we have enough solar or not?

Well, we think do, but we won’t know for sure until we get back in the water in a couple of weeks and start cruising. I can do all sorts of calculations, but the proof will be in actual use. (And yes, we have a generator as a backup.)

And that’s my main point here — the worksheets can help you estimate your usage and how watts of panels you might need, but realize that they are only estimates.

If you don’t already have one, a battery monitor is invaluable for telling you if your batteries are being sufficiently charged and how much power you do have. The battery monitor gives you the real story. Read about battery monitors here.

Another thing to remember: I’ve never met anyone who said they’d put too much solar on their boat.

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  1. helen Bell says:

    Cpt is working on a lithium battery/solar website that has a load calculator that has lots of variables to help you figure your usuage more accurately. He has included usuage demands for each appliance such as light, medium and heavy. Also a place for you to add other things that are unique to your boat. Like you said, Carolyn, it can’t be totally accurate as our needs change but it is pretty close. And knowing where you will be cruising in advance helps. We have lots of electrical things on our boat that lots of others probably do not have. Cptn designed our whole battery management system on our boat. We have been living aboard for over a year now. We do not use marinas at all. We have 1200Kw of solar,1600Ah of batteries (in two banks) and a 7Kw alternator. No generator.

    • Cptns website is a work in progress and any input would be appreciated. Also if there are items you want added please let us know. Thanks Carolyn. Love reading your articles

    • Helen – I think you need to check your numbers. 1,200 kW (1.2 MW or 1,200,000 W) of solar can’t be right. Similarly I don’t think you have a 7,000 W alternator that would be a 500 A alternator. That would have to be either direct drive or five (or more) belts.

      • Helen Bell says:

        Sorry you are right. That is 1200 watts of solar but the alternator is indeed 7kw. It puts out 300 amps max at 28v and we use a 1 1/2″ cogged timing belt to drive it. It is brushless DC and when the engine is running at 1600rpm we can still generate more than 200 amps. This allows us to fully charge our lithium batteries very quickly.

    • Helen, I think maybe autocorrect got you and changed your solar watts to kilowatts. 1200 watts is still a LOT of solar for a boat!

      • Helen Bell says:

        Sorry you are right. That is 1200 watts of solar..we run air con, fridges, freezers microwave, toaster, bread maker to name a few.

  2. An energy audit is only as good as the person who does it. It is the implementation where many of us fail and your point is well taken. In engineering that is where we add a safety margin. *grin*

    Your point is well taken about the difficulty, particularly for people just starting out, in making good estimates of usage.

    The very best thing you can do is go sailing for a season and use a battery monitor to regularly track usage. That means some other way of charging, for most of us running the main engine which isn’t all that good for it.

    Otherwise you use good engineering practice, make the very best estimates you can, get more experienced counsel, and apply a safety margin. Plan for growth with time.

    The magnitude of a safety margin is driven by the confidence in the data and the implications of over specification. Some things (battery capacity or solar output) you really can’t get too big. Others (generator size) too big can be as bad as too small.

    For solar I would not be shy. A safety factor of 2 would be a nice start and then see if you can make those panels fit your boat.

    You can’t have too much solar or too many batteries. *grin*

  3. Well, done.

    Having solarized-LED-ed our boat more than five years ago, and being statistics junkies, we were challenged by “amp hour budgeting,” and then we hit on another approach which pares well with yours.

    Firmly believing deep cycle batteries should never be discharged more than 50% (and we shoot for no more than 30%), we simply sized our solar array to restore that much charge by mid afternoon the following day.

    We assumed a latitude where we would be running the engine the least, since our ICW travel would provide plenty of alternator amps, but our island anchoring wouldn’t.

    We double checked this against the budget and found the panel wattage allowed room for usage growth even it good battery management didn’t.

    Since taking this approach we have only found it necessary to plug in at a dock for air conditioning during the worst of the summer heat.

    Solar is not free energy to start with, but the payback, given Florida and Bahamas electric rate cost avoidance has been 25 months rather than the original estimate of 60 months.– and now its free.

  4. Doug Crawford

  5. Arion McCartney says:

    I have been reading all the solar and wind posts that I come across. I see that everyone uses the rated ah’s for appliances, but those ratings are for normal running, I have not seen where anyone has tried to add in the energy that is used for the start-up of those motors which is much greater than the running energy.

  6. As always, useful advice and gets me thinking more broadly … thanks

  7. Brian Liddy

  8. I see Alex from SeaTek in marathon FL in the picture

  9. Alex at SeaTek is now installing lithium ion batteries !!

    • Helen Bell says:

      We have lithium iron phosphate batteeies (LiFePo4) we’ve been using them for over a year now..they are great

  10. LaMarr Harding says:

    Photovoltaics flat on a Bimini or deck house don’t get the full effect or their rated power.

    So the cheaper and more practical for me was just get more panels. I hate to admit that I’ve ran out of room. It’s still safer and more practical than to track the Sun

  11. Alex Miller

  12. Our two cents is get as much solar as you can comfortably fit on the boat. Once installed the energy produced is FREE! Both from a cost and maintenance perspective. This year we increased from 440 watts to 640 watts using the same area on the boat. One other thing to mention about when calculating usage / production is that solar panels will be somewhat less efficient as temperatures rise as you head to lower latitudes.

  13. As much as will fit.

  14. Tobias Stricker says:

    What you need as installed renewable energy?
    As much as you can install!
    That simple.

    Cruising boats are today swimming electronic device, just like cars are computer today. The demand is rising continuounsly. And solar panels are not this expensive any more. The limit is the space available to install them.

  15. The big issue for me was the more solar I add the more my energy needs increase. First the coffee maker… then the box fans…then the standalone freezer. Time for another 300 watter

  16. Excellent and insightful article as usual Carolyn!! We’ve also experienced the impact of heat on the solar panels and therefore producing less than expected, and of course shadowing which is difficult to control as the boat swings on anchor. We added a wind generator as well which is a good pairing for us as it helps many nights with preventing the drain from being quite so deep overnight, allowing the batteries to recover more quickly once the sun comes up in the morning. I think your most salient point is that no-one can tell you how much you really need until YOU get out there living the life – that is the only way to really know how much and what combination of energy retrieval will work for you. Thanks again for a good read!

  17. Gregory Thompson says:

    Carolyn, based on your recent trip to the Bahamas, how did you make out solar vs battery and power needs

  18. Another great post! Thanks

  19. Sandra Renwick says:

    Great information Caroline, what kind of water maker do you have and do you have air conditioning as well that you run on your boat?

    • We have a Katadyn PowerSurvivor 80E — 3.5 gallons per hour, draws 10 amps when running. Most days we ran it for 2 hours. We do not have boat air conditioning (used a portable one last summer when we were on the hard all summer).

  20. Sandra Renwick says:

    Sorry spelt your name incorrectly, Carolyn.

  21. Nathan Common

  22. John Woodworth

  23. Drew

  24. 570 watts going on.

  25. Lucky, we were just talking about this this very morning. Check it.

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