Calculating Phase angle from instantaneous voltage
You need to know the exact time difference between the instants when each of the waveform peaks.
If V1 peaks at t1 and V2 peaks at t2, or you somehow measure the time interval delta_t between their peaks, you can calculate the phase difference between them by using the following formula. T is the time period of the wave-forms.
phase difference (in degrees) = delta_t * 360 / T = (t1 - t2) * 360 / T
There are some other methods like zero crossing detection or more sophisticated techniques using FFT but I think the peak detection, like I described above, will give you a good estimate with minimal effort.
See the figure &quot;normal measurement&quot; for what I am trying to say. But sometimes your signal is noisy (see figure &quot;noisy measurement&quot;), and in that case, peak detection algorithm will have some innaccuracy depending on SNR.