I think the difference should be in frequency component but i don't know the exact difference?
power spectrum is the squared frequency spectrum (Fourier transform of the signal which has the dimension of microvolts). The resulting dimension is uV^2 (microvolts squared) (or uV^2/Hz for density). Electrical power is proportional with U^2 (P=U^2/R). Power is energy per unit of time.
Power is simply the magnitude square of the current or voltage in the signal.
A power spectrum specifically defines the distribution of this magnitude with respect to frequency.
Power spectral density function (PSD) shows the strength of the variations(energy) as a function of frequency. In other words, it shows at which frequencies variations are strong and at which frequencies variations are weak. The unit of PSD is energy per frequency(width) and you can obtain energy within a specific frequency range by integrating PSD within that frequency range. Computation of PSD is done directly by the method called FFT or computing autocorrelation function and then transforming it.
electrical power shows total energy of a power waveform.
check this:
https://math.stackexchange.com/questions/1946343/fourier-series-and-fourier-transform-amplitude-spectrum-vs-frequency-spectrum/1953123
Power spectrum shows in a figure the variations of power accross all frequencies, while electrical power shows the power of a given measure of EEG like Absolute power, Relative power in a given frequency band, like alpha.
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