i have this usb solar panel charger i am building and the solar panel is about 2by .5quot;, so i was wondering if a led light can power it, if so will one be good or would i need more? if so how many?
It could but the solar panel would produce much less energy then it took to light the LED.
An LED requires an electric current to power it. A solar panel can most definitely power an LED, but LEDs don't store any power themselves. And solar panels aren't designed to be powered, they are designed to generate electricity. Rethink your premise from the start. It is as if you were asking if the electric wires in your home or community would be powered by the light bulbs and appliances you use.
While I understand Search's answer, because it was also my first reaction, am I wrong in thinking that this is some sort of science fair project, for which you need to power a solar panel indoors as though it were being illuminated by the Sun? The question at least makes sense in that context. Solar panels provide modest power even from room lighting, sometimes calculators are powered by small panels, but that is a tiny amount of current. . You'll probably do better by asking this question (a bit more clearly) over in Engineering. It's not really an Astronomy Space question, though there may be some crossover between the two groups of users. Meanwhile, while some white LEDs can be pretty dazzling to the eye, I don't know how much actual power they are pulling down compared to what you'll need the panel to generate, but I don't see why it shouldn't work. I just don't know how many LEDs you would need. . .
Yes, shining light from a visible-light LED on a solar panel will cause the solar panel to generate electricity. There is a long-wavelength cutoff (probably in the near infrared) where photons from an infrared LED would not have sufficient energy per photon to excite the solar panel, no matter how bright the LED was. Of course the amount of electricity produced is a fraction of the power needed to power the LED. The best possible efficiency you could achieve under optimal conditions is about 30%. The typical optical power output of a single LED is around 0.3 Watt. The density of sunlight is such that the amount of sunlight falling on a 2 x .5 area perpendicular to the Sun's rays is about .3 Watts. So maybe you'd want a few LEDs, or a particularly bright LED to simulate sunlight. But you'll get something with one LED. A bright LED flashlight would work.
