Standby generators / 18 kW
18 kW standby generator: what it runs and what it burns
Generac's 18 kW Guardian is rated 18 kW on propane and 17 kW on natural gas. At full load it burns 3.02 gallons of propane an hour (1.7 at half load), so 100 gallons lasts about 33.1 hours at full load and 58.8 hours at half load.
Generac's sizing page calls 18 to 22 kW "the sweet spot" for mid-sized homes, and says a smaller home with a heat pump might step up to 18 kW. Its own product page for the 18 kW Guardian claims it can back up a five-ton air conditioner.
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 18 kW on natural gas carries 12,043.2 W running with a 4,950 W surge on top, 17 kW of its 17 kW. It has to leave out: Electric water heater, Electric range, one 8-inch element, Dishwasher, Garage door opener, 1/2 hp. On propane it carries 12,993.2 W running, 17.9 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 |
| Washing machine | 1 | 1,150 | 2,300 |
| 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) | 1.7 | 3.02 | 40.8 | 72.5 |
| Natural gas (cubic feet) | 169 | 247 | 4,056 | 5,928 |
Half load burns 56% 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 10/14/18 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 kW on natural gas and 14.9 kW on propane; on a 100 °F day at sea level, 16.3 kW and 17.3 kW. The calculator applies both when it picks a size, and the derating guide has a full table.
Smaller: 14 kW (14 kW on natural gas). Larger: 20 kW (18 kW on natural gas). Installation needs a transfer switch, a gas supply sized for full load, and the clearances in the installation manual.