An on-line UPS has radio frequency filter, rectifier filter, battery, detecting circuit, controlling circuit and trigger circuit, and its efficiency approximate a hundred percent. No matter what commercial power is failure, or its voltage is too high or too low, it is able to keep voltage output within normal range. When commercial power is abnormal, it supplies power with battery by automatic switchover, and its response time is zero millisecond. The structure of the present invention is simple and its operation is stable. It has removed the power converter of UPS on the condition that all necessary features are maintained. The costs, volume, weight and power loss decrease to a percent of conventional UPS of same power.
