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Say you have a lamp with 48 bright white led's. 16 strings, 3 led's per string at 20 mA.
At 12 V it will use .33 A continuous, or 4 W.
You need to power it at night for, say, 16 hours maximum. The batteries must store 64 W-hrs. At 12V this makes 5.3 A-hrs. A string of 6 'C' cells would be sufficient.
A charge session might be 8 hours. The PV panel must generate 64 W-hrs in that time. Hence a 12V panel must put out .7 A or more continually.
The least expensive part will be the control circuitry and light detector.
I guess I was thinking about the standalone systems we have around here... example, led's highlight a 'STOP' sign, or pedestrian crossing, or form an arrow. The solar panels cover a few square feet.
They are replacing the sodium vapour street lights with LED lights now. Each light has about 40 high power LEDs that produce a LOT more than only 4W.
They use inexpensive and reliable electricity, not solar power.
The SLOW DOWN speed limit warning signs use a solar panel of a few square feet but the LED display is off most of the time and it lights up only when you are speeding up to it.
School Crossing LED signs are the same. They blink on school days in the morning, lunch time and afternoon so they are off most of the time.
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