Generator & Inverter Sizing Calculator

How this is worked out

Every appliance has a running watts figure (what it draws continuously) and, for anything with a motor or compressor (fridges, pumps, power tools, air conditioners), a much higher starting (surge) watts figure for the brief moment it switches on. A generator or inverter needs enough continuous capacity for the sum of everything running at once, plus enough surge capacity for the single largest starting watts on top of everything else already running (the realistic worst case: your biggest motor starts while everything else is already on) — this tool adds a safety margin on top of both, since manufacturer continuous ratings are usually not meant to be run flat-out indefinitely.

Frequently asked questions

Where do I find an appliance's wattage?

Its rating plate or manual usually states running watts (or amps, which you can multiply by voltage to get watts); starting watts is less often printed, but manufacturers of motor-driven appliances sometimes quote it, or a rule of thumb of 2–3× running watts is commonly used for a rough estimate when it's not stated.

Why not just add up every appliance's starting watts?

Because appliances don't all start at the exact same instant in practice — assuming only the single largest start-up overlaps with everything else already running steadily is the standard, more realistic approach used in sizing guides, and avoids greatly oversizing (and overspending on) the generator.

Related tools: Battery Capacity & Runtime Calculator, Electricity Cost Calculator, Ohm's Law Calculator.

Source / Last verified: standard generator/inverter sizing method (sum of running watts + largest single starting watts, with a safety margin) — general electrical engineering practice, read/checked 1 October 2026.