What is a sine wave output and why does it matter?

Mains voltage takes the form of a smooth wave, the sine wave. When a UPS runs on battery, its inverterGlossary · UPS technologyInverterThe component inside a UPS that converts DC power from batteries back to usable AC power. has to recreate that wave. Simpler models produce a "modified" or "simulated" sine wave, a stepped waveform, while pure sine wave models reproduce the smooth shape of the mains. Online UPSGlossary · UPS technologyOnline (Double Conversion) UPSA UPS that continuously converts incoming AC to DC and back to AC, so the load is always fed by the inverter with clean, stable power. units as a rule deliver a pure sine wave, while Line-Interactive units come in both versions.

The difference matters because many devices are designed on the assumption that they will receive a pure sine wave. Computer power supplies with active PFC (power factorGlossary · Electrical unitsPower FactorThe ratio of real power (W) to apparent power (VA), from 0 to 1, showing how many watts a UPS can deliver per VA. correction), motors, pumps, transformers and medical equipment may run hot, hum, behave erratically or even shut down on a modified sine wave. A basic computer or router will usually work normally on a modified sine wave for the few minutes of an outage.

Practical tip: if you are powering a server, a NAS, a heating circulation pump or any motor-driven load, choose a pure sine wave UPS. A common mistake is comparing UPS units on VA alone without checking the output waveform on the datasheet, where it is usually listed as "pure sine wave".