Generator sizing guide: how many kW do you need?
Too small and the generator trips when the AC kicks on. Too big and you paid thousands for capacity you will never use. Here is the practical way to think about it.
Start with the decision, not the number
| Goal | Typical size | What runs |
|---|
| Keep the essentials alive | 10–14 kW | Fridge, lights, phones/internet, well or sump pump, garage door, a mini-split or window AC |
| Live normally, manage the AC | 18–22 kW | Essentials plus central AC (with load management), kitchen, laundry one at a time |
| Forget there is an outage | 24–26 kW+ | Everything, including multiple AC units, pool equipment, EV charging at reduced rate |
Square-foot rough guide
- Under 1,500 sq ft: 10–18 kW covers most needs
- 1,500–2,500 sq ft: 18–22 kW is the sweet spot for most coastal homes
- 2,500–3,500 sq ft: 22–24 kW
- Over 3,500 sq ft or 2+ AC systems: 24 kW and up, with a real load calculation
The loads that actually decide your size
- Central AC: the single biggest load. Starting surge matters more than running watts. Load-management modules let a 22 kW unit carry AC it could not start unmanaged.
- Well pump and sump pump: critical on the coast, high starting surge.
- Medical equipment: if this is on your list, tell the installer first — it changes the sizing and the transfer plan.
- Electric range, dryer, water heater: electric heat and appliances push size up fast; gas appliances keep it down.
- Pool, hot tub, EV: usually shed or managed during outages unless you size up deliberately.
Questions a good installer will ask you
- Square footage and number of AC units (and their tonnage)
- Gas or electric for heat, water heater, range, and dryer
- Well, septic, or sump systems that must run
- Medical or work-from-home loads that cannot drop
- Where the gas meter and electrical panel are — distance drives cost
If a quote arrives without most of these questions, it is a guess, not a sizing.