transform - Power Spectral Density computation and units

I want to make some calculs of power spectral densité of signal. For example a real voltage signal (physical unit : $V$) in time $g(t)$, its fourier transform $G(f)$ and $S_g(f)$. As far as I know, the power spectral density units for g(t) is $V^2/Hz$. However, I find in various sources the following equation to calculate the power spectral density : $S_g(f)=|G(f)|^2=G(f)G(-f)$

I do not understand this "definition" (of course the most fundamental definition of PSD is given by the Wienner-Kintchine theorem) , as it leads to a spectral density in $V^2/Hz^2$.

Regards, Mike