Generator sizing comes down to one question: what do you actually need running during an outage? Not everything in your house — the things that matter. Answer that honestly and most homes land between 10 and 26 kilowatts (kW). Answer it carelessly and you either pay thousands for capacity you'll never use, or discover mid-blizzard that your furnace and well pump can't run at the same time.
The short answer, by home and coverage
| Generator size | What it realistically covers | Typical home |
|---|---|---|
| 10 kW | Essentials: furnace, fridge, freezer, lights, Wi-Fi, garage door | Any size home, essentials-only |
| 14 kW | Essentials + one central A/C, kitchen | Up to ~2,000 sq ft |
| 18 kW | Most of the home, one A/C, laundry | ~2,000 sq ft, comfortable coverage |
| 22–26 kW | Whole home including two A/C units | 2,500–3,500 sq ft |
| 30–48 kW | Large homes, casitas, pools, EV charging | 3,500+ sq ft |
Square footage is a proxy, not a rule — a 2,800 sq ft home with gas heat and one A/C needs less than a 2,200 sq ft all-electric home with a hot tub. The real driver is your biggest electrical loads, especially air conditioning and anything with a heating element.
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Size & Price My Generator →How the math actually works
Every appliance has a running wattage (what it draws while operating) and motors also have a starting wattage — a surge of two to three times running power for the second when a compressor or pump kicks on. Sizing means adding up the running watts of everything you want on simultaneously, then making sure the generator can absorb the largest motor start on top of that.
Wattage cheat sheet
| Appliance | Running watts | Starting surge |
|---|---|---|
| Gas furnace (blower) | 600–800 | ~1,500 |
| Refrigerator / freezer | 150–400 | ~1,200 |
| Central A/C, 3-ton | 3,000–3,800 | 7,000–9,000 |
| Well pump (1 HP) | 1,000–1,500 | 3,000–4,500 |
| Sump pump | 800–1,100 | ~2,200 |
| Electric water heater | 4,000–4,500 | — |
| Electric range (one burner + oven) | 3,000–4,000 | — |
| EV charger (Level 2) | 7,000–11,500 | — |
| Lights, TV, Wi-Fi, laptops (whole home) | 500–1,000 | — |
Notice the pattern: the quiet essentials of an outage — heat, food, light, internet — total under 3,000 watts. It's the compressors and heating elements that eat capacity. That's why the honest question is "do I need A/C and the dryer during an outage, or do I need to not lose my pipes, food, and workday?"
The load-management trick installers don't always volunteer
Modern transfer switches support load management: smart modules that briefly shed low-priority circuits (a second A/C, water heater, EV charger) when the generator approaches its limit, then restore them automatically. In practice this lets an 18 kW generator behave like a whole-home system for many houses that would otherwise be quoted a 26 kW unit — a difference of roughly $1,500–$3,000 in equipment cost. Ask every bidder to price both options; how they respond also tells you who's engineering versus upselling. (See what those options cost installed in our Utah cost guide.)
Five sizing mistakes to avoid
- Sizing to square footage alone. Load list first, square footage second.
- Forgetting starting surges. The A/C that runs at 3,500 W but starts at 8,500 W is the classic reason "big enough" generators stall.
- Ignoring future loads. Planning a hot tub, basement finish, or an EV? Mention it during the site visit — capacity is much cheaper on day one.
- Oversizing "to be safe." You pay up front, you pay in fuel, and a lightly loaded big engine isn't happier for it. Load management is the smarter safety margin.
- Sizing for propane at natural-gas ratings. The same generator often produces slightly less power on natural gas than propane — check the spec sheet's fuel-specific rating, or read our fuel comparison.
How the pros do it
A good installer doesn't guess from the driveway. They'll do a load calculation based on the electrical code method — walking your panel, noting the nameplate ratings on your A/C and major appliances, and asking what matters to you in an outage. The site visit should end with a recommended size, an explanation of what it will and won't carry, and a load-management option priced alongside. If a bidder quotes a size without looking at your panel, get another bid — our installer directory lists licensed pros who do it right.
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Get My 3 Free Quotes →Sizing FAQs
What size generator do I need for a 2,000 sq ft house?
Typically 14–18 kW: 14 kW covers essentials plus one A/C; 18 kW runs most of the home comfortably. Two A/C units, electric heat, or an EV charger push you up a class — or into load management.
Will a 10 kW generator run my whole house?
Not usually the *whole* house — but it covers what an outage actually threatens: heat, refrigeration, lights, internet, and the garage door. Central A/C is what drives sizes above 10 kW.
Is it bad to oversize a generator?
It wastes $1,500–$3,000 per size class up front, burns more fuel, and light loading isn't good for big engines long-term. Right-size with load management instead.
Starting watts vs. running watts?
Motors surge 2–3× their running draw for a moment at startup. Size so the generator can carry your simultaneous running load *plus* the largest single motor start.