Worked example
A 600W load for 4 hours backup on a 12V battery bank needs about a 950 VA inverter and a 250 Ah battery.
Size your inverter
Home UPS / Inverter Sizing Calculator
What VA inverter and battery Ah you need
Inverter/UPS size
950 VA
Battery capacity
250 Ah
Assumes a 0.8 power factor with 25% headroom for VA sizing, and 80% round-trip efficiency for battery Ah. A larger surge load (e.g. a motor's starting current) may need more VA than the running-load figure alone.
How this is calculated
VA sizing. VA = (total watts ÷ 0.8 power factor) × 1.25 headroom, rounded up to the nearest 50 VA — a standard approach for sizing home inverters.
Battery Ah sizing. Watt-hours needed = load × backup hours. Battery Ah = watt-hours ÷ (voltage × 80% round-trip efficiency), accounting for inverter conversion and battery charge/discharge losses.
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Frequently asked questions
How do I add up my load in watts?
List every appliance you want backed up and its rated wattage (printed on the appliance or its box), then add them together. Common figures: LED bulb 10W, ceiling fan 75W, TV 80–150W, fridge 100–200W (running, not starting), router 15W.
Why does the calculator add 25% headroom to VA?
It's a commonly recommended safety margin so the inverter isn't run at its absolute ceiling continuously, which shortens its life and hurts its ability to handle brief surges from motor-based appliances starting up.
Should I size for full battery discharge?
No — this calculator sizes the battery Ah for your stated backup hours using typical round-trip efficiency, but repeatedly draining a lead-acid battery to 100% shortens its life. See our battery backup time calculator for the safe-vs-full-capacity distinction.
Does a motor appliance need extra sizing beyond its running wattage?
Yes — motors (like a fridge or water pump) draw a brief surge of 2-3× their running wattage on startup. If you're backing up such appliances, size the inverter with that surge in mind, not just steady running load.