What Size Generator Do I Need? (2026 Guide)
What size generator do I need? Add up your running watts plus the largest startup surge. Essentials only: 3-4 kW; with kitchen and pumps: 5-7 kW; whole home with HVAC or electric heat: 10-22 kW.
Running watts vs. starting watts
Generators carry two ratings; size for the harder one. Running watts keep loads going; starting watts cover the few-second surge a motor draws on startup — usually 2-3× running watts. Formula: recommended size = total running watts + largest startup surge (starting minus running).
Common appliance watts (running / starting)
Typical averages — check your appliance nameplate.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 600 | 2200 |
| Sump pump | 800 | 2000 |
| Well pump (1/2 hp) | 1000 | 3000 |
| Furnace blower | 750 | 2350 |
| Electric water heater | 4000 | 4000 |
| Window AC (10,000 BTU) | 1200 | 1800 |
| Microwave | 1000 | 1500 |
| Lights | 300 | 300 |
| TV + router | 300 | 300 |
| Space heater | 1500 | 1500 |
The sizing formula, step by step
1. Add the running watts
Running a fridge, sump pump, furnace blower and lights at once: 600 + 800 + 750 + 300 = 2450 W.
2. Find the largest startup surge
Motors kick on one at a time, so add the biggest single jump — the refrigerator draws 2200 W at start-up but runs at 600 W: 2200 - 600 = 1600 W.
3. Add them and round up
2450 + 1600 ≈ 4050 W, so buy one rated at least 4000 W — most people pick a 5000 W portable for headroom. Double-check with the generator size calculator.
Generator size guide: what to run
| What you run | Typical size |
|---|---|
| Essentials only — fridge, a few lights, small electronics | 3-4 kW |
| Essentials plus kitchen and pumps — microwave, sump or well pump, furnace blower | 5-7 kW |
| Whole home with HVAC or electric heat — central AC, electric water heater, electric oven | 10-22 kW |
The top band depends on region, house size and heating type — gas-heated homes in mild climates need far less.
Generator sizing tips worth following
- Match the fuel to your outage style — natural gas feeds standby units for days; propane stores cleanly; gasoline is easy to refuel but goes stale, so treat it or drain it.
- Don't overload it — keep the running total under the generator's continuous watts. A few seconds of surge is fine; sustained overload trips breakers and shortens engine life.
- Let an electrician wire whole-home units — portables just plug in, but standby units connect through a transfer switch into your panel. Never backfeed a dryer outlet; it can kill a lineman.
Also check our guides on air conditioner sizing and water heater sizing.
FAQ
Why is a generator's starting power so much higher than its running power?
Motor loads draw a startup surge — a fridge compressor, sump pump or furnace blower pulls 2-3× its running watts for a few seconds, then settles (the fridge: 600 running / 2200 starting). Size for the surge.
Should I get a portable generator or a whole-home standby generator?
A portable generator (3-10 kW) runs on gasoline or propane, starts by hand and feeds appliances by extension cord. A whole-home standby generator (10-22 kW) is wired into your panel through a transfer switch by an electrician and starts automatically on natural gas or propane.
How do I read the watts on an appliance nameplate?
Watts = volts × amps, so running watts = volts × running amps. For startup, multiply volts by surge or locked rotor amps (LRA); with only running amps shown, estimate 2-3× for motor loads. When in doubt, trust the nameplate.
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