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Q & A: solar power for night-time lights

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Most recent answer: 12/19/2016
I want to power a 30W RED BLUE WHITE LED Flood Spot light with a car battery charged by a solar panel. I would like to have the light on 24 hr a day, will a 100 watt solar panel do the job keeping the battery charged and the led light on. Alternatively I could install a on off switch and have it on only at night but that would not be a convenient. I live in south texas and we get a lot of sunny days.
- A. J. Paul (age 70)
Junction, Texas, USA

It sounds like you'll be cutting it pretty close, so a lot depends on whether it's a big problem if the light sometimes doesn't function.

Your LED draws about 2.5 A. A typical car battery can supply that for about a day. So if there's one day with very little sunshine, the light probably won't work that night. So battery capacity is one issue.

The other one is the question you raise: whether the 100 W solar panel is enough.  The LED uses 720 W-hr/day. There's some loss of energy in charging and discharging the battery, so you should expect to need more like 850 W-hr/day output from your panels. If 100 W is the panel peak output rating, you're really at the edge of what might work even on a good day.

There are several solutions. Again, what you choose depends a lot on how much reliability you need. You could just use a bigger panel and either a very big battery or two standard batteries. If you don't really need the light during the day, you could easily put in a switch governed by a small solar cell, like the ones in those little walkway lights. That way the big LED would go on only in the dark, cutting your use by a factor of about two, enough for your 100 W panel to handle most times. If you wanted to get really clever you could use two switches: one that turns it off when there's a little light (a cloudy day) and another to turn it back on when the light is very bright (sunny day, plenty of charging).

Mike W.

(published on 12/19/2016)

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