Measuring power draw of more complex loads such as switching PSU
I believe your problem is that you are assuming the current waveform is sinusoidal since you are taking only the peak value and then calculating the RMS value. In practice, since many devices use switching power supplies, the current waveform will not be sunusoidal and may have large peaks.  Since your ADC can sample fast enough, you should take many more samples over one or more cycles of the input current waveform (either 50 or 60 Hz) and calculate the RMS value from them.