Interactive tool
What size generator do I need?
Check everything you'd want running at the same time during an outage. We total the running watts, add the single largest motor-starting surge, and tell you which class of generator fits.
How generator sizing works
Every appliance has two numbers that matter. Running watts are what it draws continuously once it's going. Starting watts are the short surge an electric motor pulls in the first second or two as it spins up — typically two to four times its running draw. Resistive loads like lights, space heaters, and coffee makers have no surge at all; motor-driven loads like refrigerators, sump pumps, well pumps, and air conditioners have big ones.
Sizing is therefore a two-part check. First, add up the running watts of everything you'd realistically have on at the same time during an outage — that total must sit comfortably under the generator's rated running watts. Second, take the single largest starting surgeon your list and add it on top of the running total; that number must fit under the generator's peak (starting) rating. That's the rule the calculator above uses. You don't add every surge together, because motors almost never start in the same instant — and you can stage the big ones manually by switching them on one at a time.
Two practical margins are worth building in. Leave 10–20% of headroom over your running total: generators run quieter, cooler, and longer at partial load, and spec-sheet ratings assume sea level and mild temperatures. And if you're considering a dual-fuel unit, remember output drops roughly 10% on propane — size against the propane numbers if that's your storm fuel.
Typical appliance wattages
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 200 | 1,200 |
| Chest freezer | 150 | 1,000 |
| Furnace blower (gas heat) | 800 | 2,350 |
| Sump pump (1/2 HP) | 1,050 | 2,200 |
| Well pump (1 HP) | 2,000 | 4,000 |
| Lights (LED, whole house) | 300 | 300 |
| Microwave | 1,000 | 1,000 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Central AC (3 ton) | 3,800 | 8,000 |
| Space heater | 1,500 | 1,500 |
| Electric water heater | 4,500 | 4,500 |
| Garage door opener | 875 | 2,350 |
Typical values — check your appliance nameplates for exact figures. Motor surges vary with age and load; central AC surges can be roughly halved with a soft-start kit.
Sizing FAQ
What size generator runs a refrigerator and a furnace?
A refrigerator draws about 200 running watts (surging to roughly 1,200W on compressor start) and a gas furnace blower about 800 running watts (surging to roughly 2,350W). Together with lights and phone charging, a 3,000–4,000 running-watt generator covers them with comfortable headroom.
Do running watts or starting watts matter more?
Both, for different reasons. Your generator's running watts must cover everything you want on at the same time; its peak (starting) watts only need to cover that total plus the single largest motor surge — because in practice motors don't all start in the same instant.
Can I size a generator by square footage?
No. Two identical-size homes can differ by thousands of watts depending on whether they have a well pump, electric water heater, or central AC. A ten-minute load list — what you actually want running during an outage — beats any square-footage rule of thumb.
Do I need a 30A or 50A generator connection?
Match the connection to the generator: a 30A (L14-30) inlet and transfer switch carries up to about 7,500 running watts; a 50A (14-50) connection carries up to about 12,500. If your load math lands under 7,000W, the 30A route is cheaper and completely adequate.
Got your number? See the portable generator picks or read portable vs. standby to choose a backup strategy.