Generator Size Calculator
Wondering what size generator you need? Our free generator size calculator adds up the running watts of your essential appliances, accounts for the starting surge of the hardest-starting motor, and points you to the right standard generator size — so you can size backup power for an outage in under a minute.
Check the appliances you'll run
Adjust any wattage to match your appliance labels. Add a custom appliance at the bottom.
| Appliance | Running W | Starting W | Remove |
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Your results
Planning estimate only. Results are a starting point for sizing backup power — not a substitute for a licensed electrician's advice.
How to use this calculator
Start by checking the appliances you want to power at the same time during an outage. The list covers the usual essentials — fridge, lights, TV, chargers, microwave — plus bigger loads like a sump pump, well pump, furnace fan or window AC.
If you know your appliance's actual wattage from its nameplate or manual, type it over the preset values; typical wattages vary by model. Then add anything missing with the custom-appliance fields. Everything recalculates instantly as you check, uncheck, or edit.
Take the recommended standard size as your shopping starting point: generators come in common sizes like 3,500, 5,500 and 7,500 watts, so the tool rounds your estimated need up to the next standard size.
How it works
Running watts are the continuous power an appliance draws while it operates. Starting watts are the brief extra surge a motor needs to start up — often two to four times the running watts. A generator has to cover all your running watts plus the largest single starting surge, since motors don't all kick on at once.
This calculator uses the common surge-adder method: it sums the running watts of everything you checked, finds the biggest single surge difference (starting watts minus running watts) among your appliances, and adds that to the total. The result is your estimated need, which is then rounded up to the next standard generator size (2,000, 3,500, 5,500, 7,500, 9,500, 12,000, 15,000 or 20,000 watts).
Assumptions: preset wattages are typical values, not measured for your exact models; not all motors start simultaneously; and generator nameplate ratings can be peak rather than continuous watts. Treat everything here as a planning estimate and confirm with the generator manufacturer's specs and a licensed electrician before buying or wiring anything.
Frequently asked questions
What are running watts vs starting watts?
Running watts are the continuous power an appliance uses to keep operating. Starting watts (surge watts) are the brief spike needed to start a motor, and can be two to four times the running watts. A generator must handle the running watts of everything plus the largest single starting surge, which is why this calculator adds a surge adder on top of the running total.
What size generator do I need to run a house?
It depends on which appliances you run at the same time. A handful of essentials like a fridge, lights, TV and chargers typically fits a 3,500-watt portable generator. Add a sump pump, well pump or window AC and you are usually in the 5,500 to 7,500-watt range. Check your appliances in the calculator above for a planning estimate matched to common standard generator sizes.
Can a 7,500-watt generator run my air conditioner?
A 7,500-watt portable generator can typically handle a smaller window air conditioner along with a few other appliances, since a 10,000 BTU window AC needs about 1,200 running watts plus a large starting surge. A central AC unit usually needs far more power and is better served by a whole-home standby generator sized by a licensed electrician.
Should I get a portable or standby generator?
Portable generators (up to about 7,500 watts) are the affordable, flexible choice for running a few essentials during outages, but they need refueling and manual setup. Large portables (9,500 to 15,000 watts) can cover more circuits. Standby generators (20,000 watts and up) are permanently installed, start automatically, and can power a whole home, but they cost much more and must be installed by a licensed electrician.
How much fuel does a generator use?
Fuel use varies widely with generator size and load, so there is no single number: a generator running near full capacity burns fuel much faster than one at a light load. Check the manufacturer's specs for runtime at 50% load and plan to store extra fuel safely, in approved containers, away from the house.