An alternating current (AC) is an electric current that periodically reverses polarity and continuously varies its magnitude over time. Businesses and residences use AC power because it is less costly and more efficient for generating and transmitting than direct current (DC) power. Transformers can step up the voltage of an AC line or step it down to reduce power losses during transmission.
AC power is the kind of electrical energy residential homes receive — consumers use AC power when they plug TVs, appliances, and other devices into the wall. AC power is then converted to DC power which nearly all electronic devices use.