Best Whole-House Generator
Short answer: a whole house with central AC and electric heat or an electric water heater typically needs a 10,000–22,000W standby generator — central AC alone can draw ~3,800 running / 5,800 surge watts, and a water heater adds 4,500W. A gas-heat home covering essentials plus AC can sometimes run a large 8,000–12,000W portable with load management. Standby units require professional installation.
“Whole-house” backup means the lights, the fridge and freezer, the well or sump pump, the heating system, and — the big one — central air conditioning, all without you running out to flip loads on and off. That last item drives the size, because a central AC compressor has both a heavy running draw and a large startup surge. Here’s the honest capacity and picks by class. Product links are Amazon affiliate links and never change the advice.
Whole-house sizing at a glance
| Home profile | Approx running / surge watts | Recommended class |
|---|---|---|
| Gas heat, essentials + well pump (no AC) | ~3,350 / 5,350 | 5,000–7,500W portable |
| Gas heat, essentials + central AC, managed loads | ~7,500 / 10,000 | 9,000–12,000W large portable or small standby |
| Central AC + electric water heater + essentials | ~11,800 / 13,800 | 14,000–18,000W standby |
| All-electric home, no load management | 16,000–22,000+ | 20,000–26,000W standby |
These follow the running-watts-plus-largest-surge method. Run your exact home → Remember only one big motor starts at a time, so you add a single surge, not all of them — see running vs starting watts.
Whole-house generators by capacity class
Pick the class that matches your sizing result above. Confirm the current rated and surge watts, the fuel type, and that the unit ships with (or pairs with) a transfer switch before buying.
8,000–12,000W large portable — essentials + AC with load management:
Standby (air-cooled, ~10,000–26,000W) — true automatic whole-house backup:
We earn commission on qualifying Amazon purchases at no cost to you. See our affiliate disclosure. Wattage ratings and prices change — verify the current rated/surge watts and fuel type before buying.
Why central AC drives the size
A central air conditioner is the single heaviest load most homes have: a ~3-ton system draws roughly 3,500–4,100 running watts and surges to 5,000–6,000 watts when the compressor starts. Stack that on top of the fridge, freezer, well pump, furnace blower, and lights and the running total clears 7,000–12,000 watts — beyond any single portable run without juggling loads. That’s why true hands-off whole-house backup means a standby unit. If a single big AC is the only thing forcing you to a larger generator, a soft starter on the compressor can cut its surge by half and may let a smaller unit carry it — see running vs starting watts.
Frequently asked questions
What size generator do I need for a whole house?
With central AC and electric heat/water, usually 10,000–22,000W standby. Central AC alone is ~3,800 running / 5,800 surge; a water heater adds 4,500W. Gas-heat homes covering essentials + AC can sometimes manage a large 8,000–12,000W portable with load management.
Standby or large portable?
Standby for true automatic whole-house backup including central AC — auto-start, runs on gas/propane. A large portable (8,000–12,000W) covers most of a house through a transfer switch with manual load management.
Does it need professional installation?
Yes — a standby unit and its transfer switch wire permanently into your panel and fuel supply and must be installed by a licensed electrician to code. Never backfeed into an outlet.
Related: Generator Sizing Calculator · What Size Generator Do I Need? · Best for a Power Outage · Best for RV & Camping