Double-conversion (or On-Line) UPSs isolate equipment from raw utility power—converting power from AC to DC and back to AC again, to deliver the cleanest power and highest protection.
Double-conversion UPS systems process power twice. An input rectifier processes AC to DC, which then feeds into an output inverter that processes the DC back to AC power to send to the IT equipment. In normal operation, the UPS is always double-processing power through this combination of rectifier and inverter. If the AC input supply is out of predefined limits, the rectifier turns off, and the UPS draws current from the battery. Battery power passes through the output inverter and on to the IT equipment. The UPS stays on battery power until the AC input returns to normal tolerances (or until the battery runs out, whichever is sooner).