120vac is the accepted standard of electric current commonly used through out the U>S>. Vac speaks of ac or alternating current voltage. 240 volts is simply twice that amount. Electric lights, lighter appliances mostly use 120 volts to energize them but heavier duty appliances such as dryers required more voltage as they use more energy. Vac or alternating current as oppose to DC(Direct current )which is normally used on boats, ships, etc.