Solar Panel Wattage Estimate

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Hi Folks

For a average onboard 12v Battery what would be a good number for the number of watts supplied by solar panels for a average charge rate.

Thank You

Bruce

[CharlieJ]

My 32 watt Unisolar flexible panel supplies 2 amps at full output, which is seldom achieved. I had a controller on it that burned up. Getting a new one now- waiting for delivery. For our second controller I bought a Sunsaver 10 amp unit from Morningstar. The electronics on that one are potted in epoxy and it comes with a built in , anodised aluminum heat sink. At 50 bucks it was the best thing we found. Had a buddy in Miami who works in the electronic controller field research them for me- that's the one he found and recommended.

But on our trip this past year of 33 days aboard, the 32 watt panel supplied more than enough juice for our use. Of course, we were moving each day so we were asleep shortly after sundown and up at daylight. You don't use a lot of power like that!!

Part of that time though was offshore, with radio on full time and running lights operating at night. We had no shortage of power.


People who use more electricity than we do might find they need more.


You basically have to do a usage chart- figure how many watts or amps you are gonna use each day and get enough capacity to meet that, figuring abut half what the panels are rated for, unless you can keep them directly facing the sun all day- hard to do on a sailing vessel underway.

[James384]

Bruce,

I agree with Charlie - you need to work out a usage chart of some sort.

For example, if you have a typical group 27 deep cycle battery rated 90 amp hours reserve capacity and let's say you want to prevent normal self-discharge which I think is about 1% a day then you need to supply about 2 amp hours a day solar input. That is easily handled by a 15 watt panel even in low sun areas like where you are where it will likely give you an average output of around 3 amp hours per day.

Now say you want to use the boat on the weekends and will use 10 amp hours per day for lights, depthsouder, GPS, etc. That comes to 20 amp hours drain plus 7 amp hours self-discharge per week, or 27 amp hours per week you need to recover. Because of charging losses double it and say 50 amp hours. Now your little 15 watt solar panel that supplies more or less 21 amp hours per week is not big enough. 30 watt panel might be minimum size in that case. Because you never know if there will be a week of intense sun or a week of non-stop fog and rain in the far northeast, you may need to have a minimum of a 30 watt panel and two 90 amp hour batteries.

You also should have a charge controller for that week once a year that the sun shines every day, to prevent damage to your batteries from getting too high a voltage, especially if you go for a bigger solar panel like the 43-watt kyocera.

James
#384