Home standby generator sizes

A standby generator is wired to the house through an automatic transfer switch and runs on natural gas or propane. These pages use Generac's air-cooled Guardian line, from 10 to 26 kW, because Generac publishes ratings for both fuels and a fuel table for every size.

Ratings from Generac's spec sheets and product page; fuel from Generac's support article. See methodology.

Sizing a standby generator

Generac's sizing page gives the rule: "Total kW Required = (Sum of all running wattages) + (Highest single starting wattage burst)." It also cautions that "Two neighbors with identical floor plans could require generators with an 8kW gap or more." — square footage is only a rough guide: it puts 1,550 sq ft homes at 10 to 18 kW, 2,000 sq ft homes at 20 or 22 kW, 2,500 to 3,000 sq ft homes at 22 kW. The calculator applies the rule to your own list, on either fuel, with the altitude and temperature derate.

Rate on the fuel you have. Every size here except the 14 kW is rated lower on natural gas than on propane, so a quote that only names the propane figure overstates what the unit does on a gas line.

Propane or natural gas

On propane, a tank holds a fixed supply: at full load a 22 kW burns 3.9 gallons an hour, so 100 gallons lasts about 25.6 hours. On natural gas the supply is continuous but the rating is lower and the meter and pipe must carry the full-load flow — 327 cubic feet an hour for the same unit. The propane run-time guide covers tank sizes, and the derating guide shows how elevation and heat cut every rating here.

What each size page shows

Each page fills a whole-home list in a fixed order — essentials first, then central air, an electric water heater, a range element and laundry — and shows where that size runs out on natural gas and on propane, with the fuel it burns per hour and per day and its output at 5,000 feet and on a 100 °F day.