In the literature concerning noise it is written that as the fourier transform is not defined for a noise signal, one should first take the autocorrelation of the noise signal and then apply the fourier transform. When I look at the units of these operations this will give me a result in Volt^2/Hz. If however I have an ordinary deterministic signal I don't have to take the autocorrelation first but just the fourier transform so this will give me a result in Volt/Hz, now then how should one compare two results in different units?

candide p.s. I'm especially thinking of low frequency circuits in which noise- units are not expressed as power-units