Quick Answer
Generator size depends on which appliances you want running during an outage, not just square footage. Essentials only (fridge, sump pump, lights) typically requires 7–10 kW. Adding central air conditioning usually pushes the requirement to 14–22 kW. A whole-home system covering everything — including HVAC — typically needs 22 kW or more. A professional load calculation gives you the exact number for your home.
Key Takeaways
- 1Sizing is about what you want running during an outage — not square footage.
- 2Three common tiers: essentials (7–10 kW), most of home (14–20 kW), whole home (22+ kW).
- 3Central AC is the biggest single load and usually the deciding factor between sizing tiers.
- 4Motor startup surge (roughly 3× running wattage) is factored into every load calculation.
- 5Build in headroom for planned future loads — an EV charger or hot tub added later can outgrow an undersized unit.
- 6A professional load calculation is required before purchase; our size calculator is a useful framework before that conversation.
Start with a question, not a number
Sizing isn't really about square footage first — it's about how much of your home you want running during an outage. A household that just wants the fridge, sump pump, and a few lights running needs a much smaller unit than one that wants central air conditioning and the whole kitchen available too.
Three common tiers
Essentials only (roughly 7–10 kW): refrigerator, sump pump, some lighting and outlets, and Wi-Fi.
Most of the home (roughly 14–20 kW): essentials plus most circuits, though possibly not central air at full load.
Whole home (22 kW and up): everything running simultaneously, including HVAC — the closest to not noticing an outage happened. Use our size calculator to estimate the right kW for your appliance list.
| Size Range | Coverage Tier | What Runs During an Outage | Typical Installed Cost |
|---|---|---|---|
| 7–10 kW | Essentials only | Fridge, sump pump, lights, outlets, Wi-Fi router | $8,000–$11,000 |
| 14–20 kW | Most of home | Essentials + most circuits; AC may be limited by load | $11,000–$14,000 |
| 22–26 kW | Whole home | Everything, including central AC and kitchen appliances | $13,000–$18,000 |
What actually drives the number
Running (continuous) wattage from everything you want powered adds up first. Then, because motors draw a brief surge when they start, sizing accounts for the single largest motor's starting wattage on top of everything else already running — this is why a home with central air or a well pump often needs more capacity than raw square footage alone would suggest.
Building in a safety margin
Rough sizing estimates typically add a margin — commonly somewhere in the 15–25% range — on top of the calculated starting load, to account for real-world variation in appliance draw, aging components pulling slightly more current than spec, and any future load changes like a hot tub or EV charger added down the road.
It's worth listing out any less-common loads specifically — a home office setup, medical equipment, an extra freezer in the garage — since these are easy to forget in a rough mental estimate but add up in a formal calculation. Your installer will run a formal load calculation; our sizing calculator is a useful starting framework before that conversation.
Frequently asked questions
Sources
- NFPA 70 — National Electrical Code — Governs the electrical wiring and transfer switch requirements referenced throughout residential generator installation.
- U.S. Department of Energy — Building America Solution Center — Publishes consumer and contractor guidance on backup power options and home energy resilience.
What should you do next?
Run our generator size calculator with your actual appliance list to see the kW range that fits your home.
Estimate Your Generator Size →