What is the energy of a radiowave?

Answer 1

In general the energy conveyed by an electromagnetic wave, like a radiowave, in a second (the power P) can be evaluated by the modulus of the Poynting vector S of the wave, that is given by the vector product of the electrical and the magnetic fields vectors E and H respectively (bold letters indicates vectors)

Thus

P=| S |=| E X H |

If no charges are present there is a precise relation between the vectors E and H and the power can be evaluated also starting only from the electrical field E as

P=(eps/mu) | E |2

where eps is the dielectric constant of the medium and mu its magnetic permittivity.

Answer 2

A radiowave has low energy compared to other forms of electromagnetic radiation such as visible light or X-rays. The energy of a radiowave is determined by its frequency, with lower frequency radiowaves having less energy.