VAC is an electrical voltage in an alternating current circuit. The power system supplying homes and businesses in the United States uses AC 120 V, while in Europe the system uses AC 240 V.
Power plants supply electricity for homes and businesses with an alternating current; the current flow changes direction. In the United States, current changes direction 60 times per second, and in Europe it changes 50 times per second. This means that in the United States, the power available at a wall outlet is 120-volt, 60-cycle AC power.
The primary reason why power companies adopted the VAC system, which was invented by Nikolai Tesla, in preference to the VDC system designed by Thomas Edison, is that it is quite easy to change the voltage of the supply power by using a transformer. Power companies use extremely high voltages to transmit power over long distances, which saves them a great deal of money. If the company has 2 million watts of power available, it is possible to send a current of 1 amp at 2 million volts, which requires a reasonably thin cable and does not suffer extensive power loss due to heat. Sending 2 million amps at 1 volt would require a massive cable, and the power lost to heat would be high.