I'm getting lost with the general method of finding the power
spectrum. Here is an example:

"s1(t) - a square wave with a frequency of 1 MHz, a duty cycle of 50%
and a peak-to-peak amplitude of 2 V across a load of 50 Ù."

To find the power spectrum, I first obtained P = VI = V^2/R. I'm wondering to obtain the power spectrum whether the Fourier series of s1(t) should be calculated, and then substitute that Fourier series into P equation that I have derived. I'm getting mixed up between the signal voltage and the Fourier series of the signal. Any clarifications would be much appreciated.

"s1(t) - a square wave with a frequency of 1 MHz, a duty cycle of 50%

To find the power spectrum, I first obtained P = VI = V^2/R. I'm wondering to obtain the power spectrum whether the Fourier series of s1(t) should be calculated, and then substitute that Fourier series into P equation that I have derived. I'm getting mixed up between the signal voltage and the Fourier series of the signal. Any clarifications would be much appreciated.