UPS Battery Backup Size Calculator

UPS sizing converts the total watts of your connected devices to the required VA (Volt-Ampere) rating using a power factor.

Required VA = Total Watts / Power Factor

The standard power factor for computing equipment is 0.6–0.8. We use 0.6 for a safe margin.

Common device power consumption:

UPS sizing guide:

Runtime is a different question from sizing. This calculator answers “how big”, not “how long”. Two 1500 VA units from different makers can hold very different battery packs, so the only honest runtime number comes from the manufacturer’s chart for that model. What the runtime selector above does is nudge the recommendation up the ladder: one size at 30 minutes, two at 60. If you need a specific runtime, work backwards from the model’s published battery capacity:

Runtime (hours) ≈ Battery watt-hours × 0.85 ÷ Your load in watts

The 0.85 is inverter efficiency. A typical small tower UPS carries one 12 V 7 Ah battery, about 84 Wh, which at a 300 W load gives roughly fourteen minutes. That is enough to save your work and shut down cleanly, which is what most home units are actually for. Anything approaching an hour means an extended-runtime model with an external battery pack, not just a bigger number on the box.

Tips:

• Never load a UPS above 80% of its rated capacity
• Line-interactive UPS is best for home/office use
• Online/double-conversion UPS is best for servers
• Laser printers should never go on a UPS. Their fuser draws hundreds of watts in spikes and will trip an overload the first time it warms up