  With growing interest in multi source energy harvesting including integrated
microchips we propose a comparison of radio frequency (RF) and solar energy
sources in a typical city. Harvesting devices for RF and solar energy will be
competing for space of a compact micro or nano device as well as for
orientation with respect to the energy source. This is why it is important to
investigate importance of every source of energy and make a decision whether it
will be worthwhile to include such harvesters. We considered theoretically
possible irradiance by RF signal in different situations, typical for the
modern urban environment and compared it with ambient solar energy sources
available through the night, including moon light.
  Our estimations show that solar light energy dominates by far margin
practically all the time, even during the night, if there is a full moon in the
absence of clouds. At the same time, in the closed compartments or at the new
moon RF harvesting can be beneficial as a source of "free" energy.
