Generator size calculator
Add your appliances. Get the wattage to buy, how long a tank lasts, and what fuel costs per day — running watts, startup surge, and headroom handled.
Size your load first — we'll match generator classes to your recommended wattage below, plus the popular classes side by side.
Off-grid or battery setups need an inverter, not a generator. Size it by load type — motors and compressors pull multiples of their running draw at startup.
Running an inverter off a battery bank instead of fuel? Here's how long it lasts — after depth-of-discharge and inverter losses.
Sizing a whole bank instead of testing one battery? Use the battery bank sizer.
List the household circuits you want on backup power, one per line, like Sump pump - 800w. We'll total them and match a standard manual transfer switch.
Transfer switches are licensed-electrician work — code prohibits back-feeding, and an ATS install needs permits. Need a pro to install it? This slot is reserved for a vetted local generator installer.
Need a pro to install it?
Everything above 5,000 W, any transfer-switch connection to household circuits, and all commercial standby work is licensed-electrician territory — permits, code, and the inspection log matter as much as the genset. This slot is reserved for a vetted local generator installation and service operator. Read the facility manager's vetting checklist →
Generatorsizer connects its readers — homeowners sizing backup and facility managers planning standby — with vetted installers and maintenance contractors. Operator inquiries: this space is reserved pending terms.
How generator sizing works
Three numbers decide what you buy. Running watts — the sum of everything switched on at the same time. Starting watts — the 2–4× spike when a motor kicks on; only the largest one counts at any instant, because motors don't all start together. Headroom — generators hate running flat-out; at 100% load they run hot, loud, and short-lived, and fuel burn climbs faster than output. The 20% margin keeps you in the efficient zone and leaves room for the appliance you forgot.
Typical wattage reference
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator (Energy Star, 20 cu ft) | 150 | 600 |
| Chest freezer (10–15 cu ft) | 200 | 600 |
| Well pump, 1/2 HP | 1,000 | 2,100 |
| Sump pump, 1/2 HP | 800 | 1,300 |
| Window AC, 10,000 BTU | 900 | 1,400 |
| Central AC, 2.5 ton | 3,500 | 5,000 |
| Furnace fan, 1/3 HP | 700 | 1,400 |
| Space heater | 1,500 | 1,500 |
| LED lights, whole home | 80 | 80 |
| Microwave | 1,000 | 1,000 |
| Electric water heater | 4,500 | 4,500 |
Gas vs propane at a glance
| Gasoline | Propane | |
|---|---|---|
| Runtime on equal fuel volume | Baseline | ~25% less |
| Power output on dual-fuel models | Baseline | ~10% less |
| Storage life | 3–12 months with stabilizer | Indefinite |
| Cost per kWh (2026 U.S. averages) | ~$0.45–0.60 | ~$0.55–0.70 |
Figures from published DOE/EIA energy content: gasoline 125,000 BTU/gal, propane 91,600 BTU/gal (4.24 lb/gal). Dual-fuel generator manufacturer spec sheets corroborate the runtime and de-rate deltas.
Guides
What size generator runs a refrigerator and freezer?
The most-searched backup question, answered with real startup math — and why a 1,000 W unit beats a 4,000 W one for this job.
How many watts does a well pump use?
Starting vs running for every HP rating, plus the LRA trap that kills generators during startup.
Best 10 portable generators for home backup
Ranked by the sizing math that matters — inverter vs conventional, dual-fuel, and which class matches your load.
Find a commercial generator service
The facility manager's vetting checklist: NFPA 110 testing schedules, contract red flags, and the seven questions to ask before signing.
Battery bank sizer ↗ WeldingCalc ↗
If your load is small and daily, a LiFePO4 battery bank can replace fuel entirely — that tool sizes it.