Phase difference is the relationship in time between two waveforms of the same:

Power is a product of voltage and current, not a waveform characteristic that defines the basis for phase comparison between two signals.

Voltage is a quantity that can have a phase, but it is not the shared property that defines phase difference - two waveforms must share the same frequency for phase difference to apply.

Phase difference - measured in degrees or radians - describes how much one periodic waveform leads or lags another in time. For this measurement to be defined and consistent, both waveforms must have the same frequency; otherwise the phase relationship continuously shifts and cannot be expressed as a fixed value.

Current, like voltage, can have a phase angle, but it is not the fundamental property that must be equal between two waveforms for phase difference to be defined.