How this is calculated
One equation runs the page. Output in kilowatts times 3,412 Btu per kilowatt-hour is the energy leaving the machine each hour; divide by engine efficiency for the fuel energy going in, then by the energy in a unit of fuel.
Efficiency is the only modeled part, and it climbs with both size and load. The curve is fitted to the sheets below: 16.5 percent at full load on a 2 kW portable, 23.5 on a 22 kW standby, 28.8 on a 30 kW diesel, times 0.57 at quarter load and 0.78 at half load on a spark-ignition engine, 0.72 and 0.86 on a diesel.
Worked example, a 22 kW propane standby at half load. Eleven kilowatts out is 37,533 Btu an hour; at 18.3 percent, 204,800 Btu goes in. Propane holds 91,452 Btu per gallon and 21,591 per pound, so 2.24 gal/hr, 9.5 lb/hr, 4.3 kg/hr. A barbecue cylinder empties in two hours.
Energy contents used throughout. Gasoline 120,214 Btu/gal, diesel 137,381 and propane 91,452, from the Energy Information Administration; propane per pound, 21,591, from Quality Steel Corporation. Natural gas at the nominal 1,000 Btu per cubic foot Generac and Kohler size pipework on, against the Administration’s 1,036 for delivered gas, so real burn runs a few percent under the chart.
Read any cell as the middle of a band 20 percent either side: Generac’s 22 kW burns 3.90 gal/hr of propane flat out, Kohler’s 26RCA 3.55 while making four more kilowatts.
Quick answers
How much propane does a 22 kW generator use per hour?
Around 2.2 gal/hr at half load and 3.5 gal/hr flat out, or 9.5 and 15 pounds. Generac’s own sheet lists 2.53 and 3.90 for its 22 kW models.
How much diesel does a generator burn per hour?
Kohler’s 30REOZK sheet gives 0.9, 1.5, 2.1 and 2.6 gal/hr at quarter, half, three-quarter and full load. The 20 kW 20REOZK gives 0.6, 1.1, 1.6 and 2.1.
What load is a generator most efficient at?
On fuel alone the curve is flat from half load up. Below half it climbs steeply, and under 30 percent a diesel runs too cool to burn its fuel cleanly.
How long will a 20 lb propane cylinder run a generator?
About six hours on a 5 kW portable at half load, two hours on a 22 kW standby. The cylinder holds roughly 431,800 Btu.
Does altitude change fuel consumption?
It cuts the power the engine can make, pushing the same house load to a higher percentage of what is left. Generac quotes 3.5 percent lost per 1,000 feet, Kohler 4 percent.
What the manufacturers publish
| Machine | Fuel | 25% | 50% | 75% | 100% |
|---|---|---|---|---|---|
| Kohler 26RCA, 26 kW on propane | Propane | 64 cu ft | 85 cu ft | 111 cu ft | 129 cu ft |
| Kohler 26RCA, 24 kW on gas | Natural gas | 137 cu ft | 180 cu ft | 238 cu ft | 290 cu ft |
| Generac Guardian, 22 kW on propane | Propane | — | 2.53 gal | — | 3.90 gal |
| Generac Guardian, 19.5 kW on gas | Natural gas | — | 228 cu ft | — | 327 cu ft |
| Kohler 30REOZK, 23–31 kW standby | Diesel | 0.9 gal | 1.5 gal | 2.1 gal | 2.6 gal |
| Kohler 20REOZK, 20 kW standby | Diesel | 0.6 gal | 1.1 gal | 1.6 gal | 2.1 gal |
| Kohler 20ROZJ, 25–27 kW standby at 60 Hz | Diesel | 1.0 gal | 1.3 gal | 1.5 gal | 2.1 gal |
| Honda EU2200i, 1.8 kW rated | Gasoline | 0.12 gal | — | — | 0.30 gal |
| Champion 201393, 3.8 kW rated | Gasoline | — | 0.34 gal | — | — |
Honda and Champion figures are published run times divided into their tanks: 0.95 gallons over 3.2 and 8.1 hours, 4.7 gallons over 14 hours at half load. Kohler’s G4-315 sheet prints its own conversion: 36.39 cubic feet of propane vapor to the gallon, 8.58 to the pound.
The cliff below half load
Plot fuel per kilowatt-hour against load and the same shape appears on every sheet: a cliff on the left, a flat plain from half load rightward. The steep part is the engine spending friction, pumping and heat losses on a load too small to dilute them.
Anywhere from half load to full costs much the same fuel per unit. The reason to stop at three quarters is the headroom a motor needs to start, so that band is where a machine should normally sit.
Diesel has a second reason to stay loaded. Kohler’s paper on no-load exercising (Power Systems Topics 130): run below 30 percent of capacity for extended periods and the engine cannot sustain the temperatures that burn fuel completely. The remainder builds in the exhaust as wet stacking, fouling injectors and wearing valve guides.
- Two smaller machines alternating beat one large one loafing.
- A battery carrying the overnight base load lets the engine run hard for a few hours instead of idling all night.
Cold weather and propane vaporization
Propane leaves a tank as vapor, so the tank has to boil the liquid. Boiling takes heat, which arrives through the wetted steel from the air outside. Cold air, or a nearly empty tank, and the vapor stops arriving at the rate the engine wants.
The engine sees falling pressure, runs lean, hunts and shuts down, while frost climbs the tank.
| Tank, water capacity | At 20 °F (−7 °C) | At 60 °F (16 °C) | Biggest propane set it feeds at 20 °F |
|---|---|---|---|
| 120 gal (450 L) | 235,008 | 417,792 | about 15 kW at full load |
| 250 gal (950 L) | 406,080 | 721,920 | about 30 kW at full load |
| 500 gal (1,900 L) | 634,032 | 1,127,168 | about 48 kW at full load |
| 1,000 gal (3,800 L) | 1,088,472 | 1,978,051 | about 83 kW at full load |
A 22 kW standby at full load wants roughly 319,000 Btu an hour, so at 20 °F a 120 gallon tank cannot feed it and a 250 gallon tank can. Cylinders are weaker again, which is why a barbecue bottle frosts within minutes on anything past a camping generator.
United States vessels: 20 lb and 30 lb cylinders, 100 lb cylinders paired on a switchover regulator, 120 to 1,000 gallon tanks at 80 percent.
European vessels: 11 kg and 13 kg exchange bottles, 47 kg cylinders paired on a changeover regulator, 1,000 to 2,700 liter bulk tanks at 80 percent. Mixed bottles carry butane, which stops boiling near freezing, so a cold site wants pure propane.
Altitude, heat and the percentage
Thin air and hot air both cut engine power. Generac’s manual for its 10 to 24 kW air-cooled generators puts the loss at approximately 3.5 percent per 1,000 feet above sea level and 1 percent per 10 °F above 60 °F. The tool above uses those figures.
Kohler is harsher on the same class of machine: 4 percent per 1,000 feet above 500 feet, 2 percent per 10 °F above 60 °F, printed on the 26RCA sheet.
Hourly burn does not jump. The denominator changes. At 6,000 feet a 22 kW machine behaves like a 17.4 kW one, so a 12 kW house load that sat at 55 percent of rating now sits at 69, and the starting headroom the well pump needed has gone.
On 120 V and 240 V split-phase the derated figure is what the house load has to fit inside, motor surge on top. Work it through with the backup power calculator.
On 230 V single phase or 400 V three phase the derated figure is what the house load has to fit inside, and three phase must sit balanced across the phases. Work it through with the backup power calculator.
Turning a tank into days
Energy in the vessel divided by energy per hour gives hours, which is all the tool at the top does. Four hours morning and evening with a battery covering the gaps stretches a 500 gallon tank from seven and a half days to more than three weeks.
| Vessel | Fuel | Usable energy, Btu |
|---|---|---|
| 20 lb cylinder (9 kg) | Propane | 431,800 |
| 11 kg bottle | Propane | 523,600 |
| 100 lb cylinder (45 kg) | Propane | 2,159,100 |
| 120 gal tank at 80% | Propane | 8,779,000 |
| 500 gal tank at 80% | Propane | 36,581,000 |
| 1,000 L tank at 80% | Propane | 19,326,000 |
| 5 gal jerrycan | Gasoline | 601,100 |
| 20 L jerrycan | Diesel | 725,800 |
| 100 gal subbase tank | Diesel | 13,738,000 |
Where this chart stops
It is a planning chart, not a meter. Fuel quality, engine hours, governor tuning and the shape of the load all move the real number, and an inverter generator that drops below 3,600 rpm beats this curve at light load. Starting, warm-up and exercise cycles burn fuel that never reaches the house.
Sources
Generac Guardian 20–24 kW specification sheet, A0000937814 Rev. F, 09/28/2022, and owner’s manual for 60 Hz air-cooled generators 10 to 24 kW, A0000973303 Rev. A, 10/09/2020. Kohler sheets G4-315 (26RCA), G5-81 (20ROZJ) 6/01c, G5-435 (20REOZK) 8/23m, G5-436 (30REOZK) 8/23. Justin Loritz, Feasibility and Benefits of No-Load Exercising, Kohler Power Systems Topics 130. Honda EU2200i specifications, 0.95 gallon tank, 3.2 to 8.1 hours. Champion 201393, 4.7 gallon tank, 14 hours at half load on gasoline. Energy Information Administration, Btu content per unit. Quality Steel Corporation vaporization capacities. All read September 2026.
Go deeper
Size the machine before costing the fuel: the backup power calculator turns a load list into kilowatts, fed by the appliance wattage chart. Then what size generator backs up a whole house, whole home generator cost and standby generator against home battery.