I know this probably varies quite a bit from place to place on earth. But just some rough estimates: if I were to pull power via multiple antennae tuned to a variety of different frequencies, how much "ambient" power could I draw from the atmosphere? Perhaps I'm not asking the right question, but I'm just trying to understand how much unused energy is floating around me right now and if I could use it to say, recharge batteries slowly without an active power source.
The main problem with RF energy harvesting is the low power density (<1µW/cm^2, unless near a transmitter). Other approaches are generally more useful, though even picowatts can be enough for some applications. Some links: