power. You may be familiar them, you can usually see them in bright sunlight be squinting your eyes and looking side to side, they're the little bubbles that appear to float through your vision.
The big factories usually have acres and acres of solar nets for catching these, and trucks that ship them in to be fused into a piece of glass (which serves as a cage). They paint them black so they can't get out. BTW, don't ever scratch a solar cell, they might escape, and Fotons are what gives you sunburn, so even a small cell (with it's concentration) could, if handled improperly, give you a severe burn.
This is why you see laboratory technicians wearing the white bunny suits in their factories, the Fotons can't get through the high reflectivity of white cloth.
If you decide to set up a farm to harvest these, good luck! You might be able to stuff more of them into a solar cell than the US government allows (due to animal cruelty laws) and thereby get higher power levels, but if a USDA or SPCA inspector sees it, they might fine you for animal cruelty.