When a signal is defined in terms only of a voltage, for instance, there is no unique power associated with the stated amplitude. In this case "power" is simply reckoned in terms of the square of the signal, as this would always be proportional to the actual power delivered by that signal into a given impedance. So one might use units of V Hz for the PSD. Energy spectral density (ESD) would have units of V s Hz, since energy has units of power multiplied by time (e.g., watt-hour).