Topology first, then the gap between the VA figure and the watt figure
The best UPS units for 2026
An uninterruptible power supply keeps a machine running long enough to shut it down properly, and the specification opens on a distinction that accounts for half the price. In a standby unit the load runs from the mains until the mains fails, and a relay hands over to the inverter within a few milliseconds. A line-interactive unit adds a transformer that corrects a supply which is merely low or high without calling on the battery at all, which counts for a great deal on a weak line. An online unit runs the load from its inverter permanently and has no transfer time. IEC 62040-3 gives these arrangements formal classifications, and the maker states which one applies. Then the two capacity figures, which are not the same number. VA is apparent power and watts is real power; the watt figure is always the smaller of the two and is the one a computer's consumption has to fit inside. Runtime charts are published against a load in watts, so a runtime quoted without its load says nothing at all. The waveform on battery is either a stepped approximation or a sine wave, and some power supplies with active correction object to the former — on that point the maker of the computer, not of the UPS, is the source. Count the sockets and note which are battery-backed and which only filtered, then read the battery type and whether the pack is user-replaceable. We compare topology and its stated classification, VA and watt ratings, stated runtime at a named load, output waveform, socket count and type, battery type and replaceability, and communication port.
