Answer by Mohammad Ali for Is there a maximum number of Pis that I can power from the same wall socket?

The instructions below Assume you mean 120volts at 20 amps which is the American standard.So we want to start by calculating the approximate wattage we could theoretically draw from the wall, which is volts x amps.

120x20=2400 watts

Now we are going to calculate the number of watts a Pi uses at peak load based on your research. We use 5 volts as that is the USB standard

.75x5=3.75 watts

Now all we need to do is subtract inefficiencies in the conversion from AC to DC which is approximately 10-20% for a good power supply, we will use 15% for our calculation. So now we multiply household wattage by efficiency(.85) to get the number of watts that are actually going to reach your Pi.

2400*.85=2040 watts 

And now we decide this by the number of watts a Pi uses to see how many pi's we could theoretically connect.

2040/3.75=544 pi's

There you have it 544 pis could be connected to a single 120 volt 20 amp outlet.