What Size Whole-House Generator Do You Need? (Sizing Chart)
The list, with sourced watts
| Appliance | Running watts | Starting (surge) |
|---|---|---|
| Central AC (36,000-40,000 BTU / 3+ ton) | 6,000 | 7,200 |
| Central air conditioner (4 ton / 48,000 BTU) | 5,000 | 19,000 |
| Electric water heater (40-gal tank) | 5,500 | — |
| Electric stove (single 8-inch element) | 2,100 | — |
| Clothes dryer (electric) | 5,400 | 3,000 |
| Well pump (1 HP, 240V submersible) | 2,000 | 4,000 |
| Heat pump (whole-home, ~3 ton) | 5,400 | 7,200 |
| EV charger, Level 2 (240V) | 7,200 | — |
The arithmetic
+ largest single starting surge: 2,000W
= peak demand: 35,600W
× 1.2 headroom → recommended rating: ~42,800W
Home-standby class units from our verified catalog
Spec: manufacturer
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What size whole-house generator do I need?
For every big load at once on an all-electric home: ~33,600W running, 35,600W 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)
- TheGreenWatt - AC wattage by tonnage chart (vendor)
- UDPOWER - 4-ton AC watts guide (with Goodman RLA/LRA example) (vendor)
- Midwest Electric Cooperative Corp. wattage page (citing U.S. DOE EERE) (utility)
- Champion Power Equipment 'Choosing a Generator' wattage flyer (manufacturer)
- Franklin Electric AIM Manual, 60 Hz Submersible Systems (Tables 3 + 29) (manufacturer)
- Erayak Power well-pump generator sizing chart (vendor)
- SizeMyGenerator well-pump starting-watts guide (figures derived from Franklin motor tables) (vendor)
- U.S. DOE Alternative Fuels Data Center - EV charging infrastructure (Level 2 specs) (gov)
Figures are conservative sizing values chosen from the range these sources report. Your appliance's nameplate is authoritative.