Solar, Batteries & Backup Power
How to Size a Whole-House Generator
Oversizing wastes money; undersizing means the generator can't start your AC or well pump. Sizing starts with deciding what must run.
Updated 2026-10-15
Essential circuits vs whole home
An essentials system powers a refrigerator, furnace blower, sump or well pump, lights, and outlets — often 10–14 kW is plenty. A whole-home system includes central AC, electric cooking, and laundry, which commonly pushes into the 20–26 kW range for typical houses.
Starting loads matter most
Motors draw several times their running current to start:
- Central AC compressors (soft-start kits reduce surge)
- Well pumps
- Sump pumps
- Large refrigeration and freezers
Load management changes the math
Modern automatic transfer switches can shed big loads — AC, dryers, EV chargers — and restore them when capacity frees up. That lets a smaller generator serve a whole home without overload, which saves on unit cost and fuel.
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Frequently asked questions
Natural gas or propane?
Natural gas is convenient where available but delivers slightly less output than propane for the same unit. Propane requires a tank sized for your expected runtime.
How long can a standby generator run?
On natural gas, as long as supply holds; manufacturers recommend periodic checks during extended runs. On propane, runtime depends on tank size and load.
Do I need a permit?
Yes — typically electrical, sometimes gas/mechanical and zoning for placement. Your installer usually handles all of them.
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This guide is general information, not engineering or legal advice. Electrical codes are adopted and amended locally — consult a licensed electrician and your local building department for your project.