Generator Size Calculator
What size generator do you need? Add the appliances you want during an outage and calculate the running watts, largest starting surge and recommended generator capacity.
Reviewed by MyEnergyToolkit Editorial Team · Updated July 24, 2026 · Methodology & sources
| Appliance | Running W | Starting W | Remove |
|---|
Recommended generator size
0 kW
covers 0 W required
Estimates for planning only. Always verify appliance nameplates against your local electrical code and consult a licensed electrician — never connect a generator to house wiring without a transfer switch.
Generator size chart for a house
These ranges are planning examples, not a replacement for a load calculation. Electric heat, central air conditioning, well pumps and electric water heaters can move a home into a much larger class.
| Backup goal | Typical generator size | Typical loads |
|---|---|---|
| Small essentials | 2,000–3,500 W | Refrigerator, lights, internet and chargers |
| Home essentials | 5,000–7,500 W | Essentials plus furnace blower, sump pump or microwave |
| Managed home backup | 8,000–12,000 W | Several circuits with large loads used one at a time |
| Whole-house backup | 14,000–24,000+ W | Most household circuits, potentially including central AC |
What size generator can run a house?
A 5,000–7,500 W portable generator can often cover carefully selected essentials, while whole-house standby systems commonly start around 14,000 W and rise beyond 24,000 W. Service size alone does not determine generator size: a 200 A electrical service describes the panel's maximum capacity, not the loads that operate simultaneously. Add the actual loads above or arrange a professional load calculation.
How this generator size calculator works
Generators have to handle two numbers: the continuous (running) watts of everything switched on, and the brief starting surge when a motor-driven appliance kicks in. Since appliances rarely all start at once, the standard method is: required watts = total running watts + the single largest surge (starting minus running watts of your biggest motor load).
The recommendation then adds 25% headroom — running a generator flat-out shortens its life and leaves no margin for the loads you forgot. Find running and starting watts on each appliance's nameplate or manual, or use our appliance wattage chart, which lists both running and surge watts for 60+ common appliances — exactly what generator sizing needs.
For worked household examples, see the whole-house generator sizing guide. Never connect a portable generator directly to house wiring without an approved transfer switch or interlock installed according to local requirements.
Generator sizing FAQ
What size generator do I need for my house?
Add the running watts of everything you want on at once, then add the single largest surge. For essentials — fridge, lights, TV, chargers, a sump pump — most homes land between 3,500 and 7,500 watts. Whole-house backup with AC typically needs 10,000–24,000 watts.
What's the difference between running watts and starting watts?
Running (rated) watts is continuous draw. Starting (surge) watts is the brief extra power motor appliances need to spin up — often 2–3× running for fridges, pumps, and ACs. A generator must cover total running watts plus the largest single surge.
Why add 25% headroom?
Running a generator at full load shortens its life, burns more fuel per kWh, and leaves nothing for loads you forgot. Sizing at ~80% of capacity keeps the engine efficient and covers label inaccuracies.
Can I run a refrigerator and AC on the same generator?
Yes, if it covers both running loads plus the largest surge — but stagger their startup so the surges don't stack. A soft-starter kit on the AC can cut its surge dramatically.
Is it safe to size and connect a generator myself?
Sizing is fine to estimate; connecting is not DIY. Backfeeding a home without a transfer switch is illegal and can kill utility workers. Plan here, then have a licensed electrician install a transfer switch or interlock.