What Size Whole-House Generator Do You Need? (Sizing Chart)

Last verified 2026-08-28 · every wattage cites its source · methodology

Backing up an all-electric house — central AC, electric water heater, range, dryer, well pump, heat pump, EV charging — takes about 24,400 running watts with 26,200W available at startup. That is home-standby territory (14–26kW installed units), not portable-generator territory; a portable covers a subset of these loads at a time.

The list, with sourced watts

Appliance	Running watts	Starting (surge)
Central AC (36,000-40,000 BTU / 3+ ton)	6,000	7,200
Electric water heater (40-gal tank)	5,500	—
Electric stove (single 8-inch element)	2,100	—
Clothes dryer (electric)	5,400	3,000
Heat pump (whole-home, ~3 ton)	5,400	7,200

The arithmetic

Running total: 24,400W
+ largest single starting surge: 1,800W
= peak demand: 26,200W
× 1.2 headroom → recommended rating: ~31,500W

Home-standby class units from our verified catalog

Generac Guardian 26kW (7290)
22,500W rated · natural gas / liquid propane (standby)
Spec: manufacturer

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Westinghouse WGen14500DFc
14,500W rated · 18,000W starting · dual fuel (gasoline/propane)
Spec: manufacturer

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Adjust this list in the calculator →

What size whole-house generator do I need?

For every big load at once on an all-electric home: ~24,400W running, 26,200W peak — a 14–26kW standby unit territory. Load-management (locking out AC or water heater while another big load runs) lets a smaller unit cover the same house.

Whole-house standby vs portable — what is the real difference?

A standby unit is permanently installed, starts automatically, and sustains 10kW+ for days on natural gas or propane. Portables top out around 9–12kW sustained and need refueling; they back up circuits, not the whole panel.

Why do starting watts matter?

Central AC compressors and well pumps briefly draw 2–4× their running watts. The unit must cover the running total plus the largest single surge.

Sources

Wattage Worksheet '3R0WATTGUIDE' (generator reference guide, Home Depot-hosted PDF) (vendor)
A-iPower Wattage Chart (Sam's Club-hosted PDF) (vendor)
Howell-Oregon Electric Cooperative generator sizing chart (utility)
Midwest Electric Cooperative Corp. wattage page (citing U.S. DOE EERE) (utility)
Champion Power Equipment 'Choosing a Generator' wattage flyer (manufacturer)

Figures are conservative sizing values chosen from the range these sources report. Your appliance's nameplate is authoritative.