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.

Short answer

Add up the running watts of everything you need on at once, then add only the single largest motor startup surge — a fridge, well pump, AC, or furnace blower briefly pulls 2–3× its running watts when it starts, but they rarely start together, so you size for one surge, not all of them summed. Practical results: essentials (fridge, furnace fan, sump, a few lights) usually fit an ~8 kW portable generator; add a 3-ton central AC and you're into ~26 kW standby territory. A home battery must clear two bars — enough stored kWh and enough inverter surge to start those motors. And the rule that actually saves lives: never run a portable generator in a garage, basement, or near a window — carbon monoxide kills in minutes. Pick your loads below for an exact size and a 20-year cost comparison.

Last updated June 10, 2026 · methodology & sources

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.

Frequently asked questions

What size generator do I need to run my house?

Add up the running watts of everything you need on at once, then add only the single largest motor startup surge. Essentials (fridge, furnace fan, sump, a few lights) usually fit an ~8 kW portable; add a 3-ton central AC and you're into ~26 kW standby territory. Pick your loads above for an exact size.

Do I add up all the startup surges when sizing a generator?

No. Motors draw 2 to 3 times their running watts for a second or two when they start, but they almost never start together — so the math is all your running watts + the single biggest surge. Sum every surge and you'll buy twice the generator you need; ignore surge and it browns out when the well pump kicks on.

Can a home battery power my whole house in an outage?

Only if it clears two bars: enough stored kWh to run your loads, and enough inverter surge to start those motors. A battery can hold plenty of energy and still fail to start a well pump because the inverter can't deliver the surge. This tool checks both.

Is it safe to run a portable generator in the garage?

No. Never run a portable generator in a garage, basement, or near a window — carbon monoxide kills in minutes. The tool puts the CO rules in front of every portable recommendation, because on this one prominence is a safety feature.