The letters VAC stand for “volts of alternating current” (AC). In the United States, 110 to 120 volts is the standard voltage that comes out of a wall socket.
The listed AC voltage is an average of the actual voltage. This means that 120 volts AC actually changes from 0 to 170 volts in one direction and back to 0 to 170 volts in the other direction. The RMS method, which stands for root-mean-square, is used to figure out this average, which is the same amount of power that 120 volts direct current (DC) delivers. In the United States, this cycle of alternating current happens 60 times per second. In Europe and most of Asia, the current changes 50 times per second.
