UPS Battery Sizing Calculator

UPS units are rated in volt-amps (VA), not watts, and the two only match up for purely resistive loads — computers, monitors, and networking gear all draw power in bursts that make the effective (apparent) power higher than the wattage on the label. This calculator accounts for that by dividing your total wattage (computer plus monitor plus router plus other equipment) by an assumed 0.6 power factor typical of switching power supplies, which is why the minimum VA rating always comes out noticeably higher than the raw wattage sum. It then adds 25% headroom on top of that minimum, because running a UPS consistently near its rated ceiling shortens battery life and risks overload alarms, and rounds the result up to the nearest UPS model size actually sold (350, 425, 550VA, and so on up through 3000VA).

Required battery capacity in watt-hours is calculated separately from your desired runtime in minutes, factoring in a 90% inverter efficiency loss as DC battery power gets converted to AC output. The "estimated actual runtime" output, though, isn't derived from that battery-Wh figure — it instead works backward from the typical battery capacity built into the recommended UPS model, using an industry rule of thumb of about 45 Wh per 500VA of rated capacity, which is why bumping up the desired runtime input can raise the recommended UPS size without necessarily matching the runtime you asked for exactly; treat both runtime numbers as ballpark estimates, since real UPS battery capacity varies by manufacturer and battery age. Cost is a rough $0.10-per-VA street-price estimate for consumer units, not a specific product quote.