Power supply Harmonics calculation using FFT
Leaving aside the mismatch in power, for a minute:

Your sampling rate is 400 Hz.

Assuming you live in a 50 Hz country, the frequencies you are interested in are the 5th harmonic at 250 Hz, the 7th harmonic at 350Hz, and so on.

Your sampling rate is not fast enough. Your sampling rate must be at least twice the frequency of the highest frequency signal you are interested in.

As an example, if you want to detect up to the 19th harmonic (1,950 Hz) your sampling frequency must be at least 3,900 Hz.

Fix that, then try your calculations again and see if you get sensible results.