There are three basic types of UPSs for modern use. The first, known as “Off-line” or “Stand-by” UPS, passes incoming mains power through to the output receptacles. It continually monitors the line for a major voltage sag or complete power failure. When either is recognized, the UPS will then switch on its inverter (the part that produces output alternating current) and supply output power from the battery or batteries until the sag returns to a safe operating voltage or the main voltage has been restored. The second type would be a “Line Interactive” UPS. These units are fitted with an AVR (Automatic Voltage Regulators) which will boost and buck power during sag and swell (overvoltage) conditions, as well. UPSs with AVRs do not go to the battery as often, thereby saving on the life of the battery. There is, by definition, a momentary power loss during either type switching over to the battery, typically 2-4 milliseconds, which is insignificant to most loads.
The third type is known, as an “On-line” design because its inverter is always “on-line.” In this design, all incoming power is converted to direct current, which both fills up the batteries and feeds the inverter. The inverter is constantly producing fresh alternating current to supply the load. When the main power is interrupted, the batteries are still holding up the incoming ‘DC supply to the inverter, so the load continues to be supplied with absolutely no break whatsoever. The inverter is a constant-duty inverter, so this costs a little more. In any reputable “on-line” UPS, there is also a fail-safe mechanism known as a “Static Bypass,” which, upon detecting a failure in the inverter, will throw the load on the mercy of the main supply, rather than letting it drop altogether. This design inherently cleans the supply by virtue of reducing it to DC, then reconverting it to fresh AC.