How we size a generator
All generator sizes on this site use one shared load sheet: the running load comes from the optional dwelling calculation in NEC 220.83, followed by the largest motor start and the fuel derate. For your permit, the electrician's own load calculation is what governs.
The load sheet, shown line by line
Worked through: a 2,000 sq ft home heated by gas with a 3-ton central air conditioner.
| Line | Rule | 2,000 sq ft example |
|---|---|---|
| General lighting and outlets | 3 VA per sq ft | 6,000 VA |
| Small appliance circuits | 2 kitchen circuits at 1,500 VA | 3,000 VA |
| Laundry circuit | 1 circuit | 1,500 VA |
| Fixed appliances | nameplate watts of each one you pick | none picked |
| Subtotal of the lines above | added | 10,500 VA |
| Counted: the first 8,000 VA of the subtotal | at 100% | 8,000 VA |
| Counted: the rest of the subtotal | at 40% | 1,000 VA |
| Counted: heating or cooling, whichever is larger | at 100% | 4,300 VA |
| Running load | the counted lines added together, with 1,000 VA read as 1 kW | 13.3 kW |
Citing NEC 220.83 by section, this page doesn't reproduce the code text, which is NFPA's.
Starting watt figures
For one or two seconds while starting, a motor pulls two to three times the watts it runs at. The largest motor start is added to the running load on the sheet, because motors seldom start together. For the example, the central ac, 3 ton, contributes 2.5 kW, giving a peak of 15.8 kW. With a soft starter on the air conditioner, about 35% of that surge remains.
The running load must be no more than 80% of the generator's rating, while the peak must be carried at full rating. Whichever of these two needs more power sets the size.
Derate
Natural gas output is lower for a standby generator than propane output. The sheet applies 89% of the propane rating, based on the manufacturers' spec sheets. Once you're above 500 ft, every extra 1,000 ft costs the engine about 3.5% of its power. That example calls for 18.7 kW, so a generator of 22 kW comes next.
Gasoline is what portables are rated for, and unless the manufacturer lists a portable's starting watts, the sheet counts them at 125% of its running rating.
Cost figures and sources
| Figure | Source | Period |
|---|---|---|
| Appliance running and starting watts | Wattage chart from Champion Power Equipment; EV charger running on 32 A and 240 V | as published |
| Fuel per hour by size | Generac spec sheets, half and full load | as published |
| Natural gas costs $21.92 per thousand cubic feet on average in the US | EIA residential natural gas prices by state | March to August 2026 |
| Propane, $2.73 a gallon, US average | EIA heating oil and propane update | October 26, 2026 |
| Gasoline, $4.27 a gallon | EIA gasoline and diesel update | October 26, 2026 |
| Electrician wage, $34.37 an hour, US average | BLS Occupational Employment and Wage Statistics | May 2025 |
| Outage minutes a year by state | EIA Electric Power Annual, table 11.4 (SAIDI) | 2025 |
| Generator list prices | Generac and Kohler price pages | read 2026-11-01 |
| Install labor, $3,000 to $5,500 | Install ranges published by Home Depot, Angi and PowerOutage.us | as published |
| Sound levels | Manufacturers' spec sheets | as published |
Each figure is an estimate used for planning. Your site, gas run and electrician all affect the cost.