Standby generators / 20 kW
20 kW standby generator: what it runs and what it burns
Generac's 20 kW Guardian is rated 20 kW on propane and 18 kW on natural gas. At full load it burns 3.74 gallons of propane an hour (2.36 at half load), so 100 gallons lasts about 26.7 hours at full load and 42.4 hours at half load.
The 20 kW burns less natural gas at half load (164 cubic feet an hour) than the 18 kW does (169), by Generac's fuel table, though more at full load. Generac's sizing page puts a typical 2,000 sq ft home at 20 or 22 kW for whole-home coverage.
What it carries
Filling a whole-home list in a fixed order — essentials first (refrigerator, sump pump, furnace fan, well pump, CPAP, lights, electronics, microwave, coffee maker), then central air (2 ton), an electric water heater, a range element, washer, dishwasher, garage door and a desktop computer — and adding the single biggest starting surge to the running total as Generac's formula does, the 20 kW on natural gas carries 12,993.2 W running with a 4,950 W surge on top, 17.9 kW of its 18 kW. It has to leave out: Electric water heater, Washing machine, Dishwasher, Garage door opener, 1/2 hp. On propane it carries 14,593.2 W running, 19.5 kW with the surge.
| Load (natural gas list) | Qty | Running W | Extra to start |
|---|---|---|---|
| Refrigerator/freezer | 1 | 700 | 2,200 |
| Sump pump, 1/2 hp | 1 | 1,050 | 2,150 |
| Furnace fan blower, 1/3 hp | 1 | 700 | 1,400 |
| Well pump, 1/2 hp | 1 | 1,000 | 2,100 |
| CPAP machine (ResMed AirSense 11) | 1 | 73.2 | — |
| LED light bulb, 15 W | 6 | 90 | — |
| Cell phone charger | 2 | 40 | — |
| Television, LED | 1 | 80 | — |
| Laptop | 1 | 60 | — |
| Microwave, 1,000 W | 1 | 1,000 | — |
| Coffee maker | 1 | 1,500 | — |
| Central air conditioner, 24,000 BTU (2 ton) | 1 | 3,800 | 4,950 |
| Electric range, one 8-inch element | 1 | 2,100 | — |
| Desktop computer with monitor | 1 | 800 | — |
Central air with the basics alone needs 9.6 kW by the brochure figures — inside this unit's natural gas rating. Generac's sizing page warns that a 3-ton system "typically requires about 3.5kW to 5kW of running wattage" and can need 15 to 18 kW to start, far more than the brochure's 2-ton row; check the compressor's locked-rotor amps with an installer.
Fuel use
| Fuel | Half load, per hour | Full load, per hour | Half load, per day | Full load, per day |
|---|---|---|---|---|
| Propane (gallons) | 2.36 | 3.74 | 56.6 | 89.8 |
| Natural gas (cubic feet) | 164 | 287 | 3,936 | 6,888 |
Half load burns 63% as much propane as full load, not 50%, so halving the load does not double the run time. Per-day figures assume the load holds steady for 24 hours; a real outage swings between the two. Generac's continuous-use guidance asks for a shutdown of at least 30 minutes after every 24 hours of running. The propane run-time guide works through a tank.
Fuel: Generac Support, How much fuel does my home standby generator use (Guardian Series table, Feb 12, 2026). Ratings: Generac Guardian 20/22/24 kW spec sheet.
Altitude and heat
Generac's spec sheet: "Maximum power decreases approximately 3.5% for each 1,000 ft (304.8 m) above sea level and approximately 1% for each 10 °F (6 °C) above 60 °F (16 °C)." At 5,000 feet that leaves 14.9 kW on natural gas and 16.5 kW on propane; on a 100 °F day at sea level, 17.3 kW and 19.2 kW. The calculator applies both when it picks a size, and the derating guide has a full table.
Smaller: 18 kW (17 kW on natural gas). Larger: 22 kW (19.5 kW on natural gas). Installation needs a transfer switch, a gas supply sized for full load, and the clearances in the installation manual.