Ac or alternating current is type of power delivery via electricty in which the voltage is oscilated, usually using some form of sin wave.
Power delivery is difficult to calculate for AC in comparison to DC. For DC, the power delivery is calculated via
P = E^2 / R
For AC, however, we substitute E with the root mean square (RMS)
The RMS can be any value for different waves, however for the most common type of wave, the sin wave, we can simply multiply the peak voltage by 0.707. For example
Sin wave with a peak voltage of 17V has an RMS of 0.717 * 17 = 12V
To convert from the Power delivery of an AC line back to the peak power, simply multiply the value by 1.414. For example
A 120V AC power line has a peak voltage of 120 * 1.414 = 169.7V