Load Matters More Than Generator Size
A generator burns fuel in proportion to what you plug in, plus a fixed amount just to keep the engine turning. Generac's spec sheet for its 22 kW air-cooled standby unit lists 2.16 gallons of propane an hour at half load and 3.68 at full load. Run the line back to zero and the engine burns about 0.64 gallons an hour before it powers anything.
That idle cost is why an oversized generator is expensive in an outage. At a 6 kW average — a realistic whole-house load without electric heat — the 22 kW unit burns about 1.5 gallons an hour, or 35 a day. A 9 kW unit carrying the same essentials would burn noticeably less, simply because it's a smaller engine.
Runtime on Common Propane Supplies
A 22 kW standby generator at a 6 kW average, starting from an 80% gauge and stopping at 20%:
- 100-lb cylinder (23.6 gal): about 16 hours
- 250-gallon tank (150 usable): about 4 days
- 500-gallon tank (300 usable): about 8.5 days
- 1,000-gallon tank (600 usable): about 17 days
A 3,500 W portable at half load (about 1.6 kW) runs roughly 10.5 hours on a 20-lb grill tank, 21 hours on a 40-lb and 52 hours on a 100-lb cylinder. More runtimes by generator and tank are in how long a propane tank runs a generator.
Cold Weather: When the Tank Can't Keep Up
Propane leaves a tank as vapor, and it boils off the liquid using heat from the outside air. The colder it is, and the emptier the tank, the less vapor it can supply per hour — regardless of how many gallons are left. A full 100-lb cylinder can deliver about 113,000 BTU an hour at 0°F and 167,000 at 20°F (Flame Engineering's chart), and only about half that once it's half empty.
A 22 kW standby at half load needs about 198,000 BTU an hour. A single 100-lb cylinder can't supply that in freezing weather, so the generator surges, loses output or shuts down with fuel still in the tank. Manifold several cylinders together or use a bulk tank: a 500-gallon tank supplies over 260,000 BTU an hour even at 0°F. Small grill tanks have the same problem on large portables; in a deep freeze, a 20-lb cylinder struggles to feed even a 3,500 W unit at half load.
Propane vs Natural Gas vs Gasoline
On piped natural gas there's no tank to run dry, and at typical prices it costs less per day: the 22 kW unit at 6 kW burns about 3,500 cubic feet a day, roughly $62 at $1.70 a therm, against about $88 of propane at $2.50 a gallon. Gasoline portables are cheapest to buy but the hardest to keep fueled: a typical 3,500 W unit runs about 9 hours on its 3.4-gallon tank at half load, and stored gasoline goes stale in months without stabilizer.
Never run a portable generator indoors, in a garage or near windows; carbon monoxide from portable generators kills dozens of people in the U.S. every year.