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Sizing a Solar Panel & Battery for an LED Light

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Drunk Dumbass

New Member
Hi everyone! I'm new to the world of electronics but I have an idea for a project and I was wondering if one of you old salts wouldn’t mind reviewing my calculations and let me know how they look. This is a pretty simple project so it should be a quick easy review for those of you who know what you’re doing.

I have a 12 volt, 10 LED light that I would like to hook up to a battery & solar charger with a photocell switch so that the light will come on at night and shut off during the day.

The current draw for the light is .08 Amps. So 12 volts X .08 Amps = .96 Watts

I figure during the winter it will get 8 hours of good sunlight for charging and then the light will stay on 12 hours at night. There will be some hours in the morning and evening when the sunlight won’t bright enough for charging, or dark enough to turn the LED on, so I’m not counting those hours.

So the amount of energy the light uses each night would be .96 Watts X 12 hours = 11.52 Watt-hours.

Size of the solar panel needed would be at least 1.44 Watts, that’s 11.52 Watt-hours / 8 hours.

And the battery would need to be at least a 12v .96ah battery.

So a 5 watt solar panel and a 12v 5ah battery should be big enough to give plenty of safety margin to cover cloudy days and such.

Does this look okay to everyone or is it way off base?
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Well-Known Member
You can check the online sources for "solar insolation" for your state and seasons ie winter.

At higher latitudes a solar panel might only get average 3 hours per day at full output, maybe less if you get a rainy spell, and a 5w panel might not make 5w with a weak winter sun.

I would worst case everything, ie allow for 15 hours of light on = 15x0.08A = 1.2ah
a battery good for 4 full days no sun = min 6ah

So a 5 watt panel at 3 hours effective duty is 15wh, sounds ok but doesn't quite have the ability to power the energy requirements AND recharge the battery. It might cut it provided you dont have a week of solid winter rain etc.

Since the next alternative is probably a much more expensive 10watt panel I would probably try the 5w panel and a slightly larger battery, say a 10or 12 ah battery.

You may need a charge controller since the battery will be charged during the day and the load is at night (which is always a worst case scenario). You can make a simple power zener with a 13.2v zener diode and a transistor on a heatsink if you don't want the complexity of a proper charge controller.
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