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Start with the real load you need to protect. Add up the wattage of the devices you want on backup - a typical desktop PC and monitor draw roughly 150-400W, a router and a few peripherals add a little more - then add 20-30% headroom and never plan to run a UPS above about 80% of its rated watts. Sizing on watts, not the marketing VA number, is the single most common mistake: a UPS carries both a VA figure and a lower watt figure (VA multiplied by its power factor, usually 0.6-0.9), and the watt figure is what your equipment actually consumes.
Once the capacity is right, match the topology and waveform to how sensitive your equipment is and how long you need to stay up. Line-interactive units with AVR suit most home and small-office PCs; online double-conversion units suit servers, NAS and anything that cannot tolerate a switchover gap. Pure sine wave output matters for modern PC power supplies with active PFC. The five factors below break down each decision. Evetech stocks UPS systems from brands such as APC, Mecer and RCT across these tiers.