To summarise, a UPS transforms input alternating current (AC) to direct current (DC) in order to charge the backup battery and operate the inverter. The inverter then transforms this power back to alternating current and feeds it to the load. When the power goes out or there is a surge, spike, or other problem, the power can be taken from the battery by the inverter, which converts it back to alternating current (AC), and is used to ensure you have a continuous power supply.