What size generator (or battery) do you actually need?

Pick the things you need to keep running in an outage. You'll get the minimum generator size — sized the way electricians do it, running watts plus the single largest startup surge, not the naive sum — plus a battery option with a real inverter-surge check and a 20-year cost comparison.

1 · What do you need to power?

Quick starts:

2 · Cost & battery assumptions (editable)

The one rule that gets backup power sizing wrong

Electric motors — your fridge compressor, well pump, furnace blower, AC — draw 2 to 3 times their running watts for a second or two when they start. That surge, not the running load, is what decides whether your generator stalls. But you don't add up every motor's surge: they almost never start at the same instant, so the right math is all your running watts + the single biggest surge. Add all the surges and you'll buy twice the generator you need; ignore surge entirely and you'll buy one that browns out the moment the well pump kicks on. This tool does it the electrician's way.

The part that actually kills people: carbon monoxide. A portable generator run in a garage or near a window has killed people in minutes. This tool puts the CO rules in front of every portable recommendation, because on this one, prominence is a safety feature — not an afterthought.

Battery isn't just "enough kWh"

A home battery can hold plenty of energy and still fail to start your well pump, because the inverter can't deliver the surge. This tool checks both — capacity and the inverter surge — so you don't find out during the storm. Heating with a heat pump in the cold? See the heat pump sizing tool.