Generator Fuel Calculator Widget

Give generator buyers and site managers a running-cost estimate. They enter the generator's rating, the average load and the fuel, and optionally the tank size, price and the full-load consumption from the data sheet, and see fuel per hour and per day, fuel per kWh, runtime per tank and the cost.

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<iframe src="https://a2z.tools/embed/w/generator-fuel-calculator" title="Generator Fuel Calculator by A2Z Tools" width="100%" height="840" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

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How it works

Generator fuel use is close to a straight line in load: a fixed amount just to keep the engine turning, plus an amount proportional to output. HOMER's fuel-curve model writes this as F = F0 x rated kW + F1 x output kW, and its documentation's worked example uses F0 = 0.033 and F1 = 0.273 litres per hour per kW for a diesel set - 0.306 L per kWh at full load. A 20 kW diesel at 75% load therefore burns 0.033 x 20 + 0.273 x 15 = 4.76 L an hour. For petrol, LPG and natural gas the same part-load shape is used, with full-load consumption worked out from each fuel's energy content (US DOE and EIA figures) and an assumed 25% full-load efficiency for spark-ignition engines - clearly an estimate. If you enter the full-load figure from your generator's data sheet, it replaces the estimate and only the curve shape is used. Runtime is the tank divided by hourly use, and cost per kWh shows how badly a lightly loaded set performs.

Calculation method

  • Diesel: F (L/h) = 0.033 x rated kW + 0.273 x output kW (HOMER fuel-curve form, documentation example values)
  • Same shape for any fuel: F = full-load F x (a + (1 - a) x load), a = 0.033 / 0.306 = 0.108
  • Full-load use, other fuels = rated kW x 3,600 kJ/kWh / (0.25 x energy per unit); petrol 112,114 Btu/gal, propane 84,250 Btu/gal (AFDC lower heating values), natural gas 1,037 Btu/cu ft (EIA)
  • Runtime = tank / F; cost per hour = F x price; cost per kWh = cost per hour / output kW

Worked examples

20 kW diesel at 75% load

Inputs: 20 kW; 75%; diesel; 200 L tank; 1.60 per litre

Result: 4.76 L/h; 114.1 L a day; 42.1 h per tank; 7.61 an hour

0.033 x 20 + 0.273 x 15 = 4.755 L/h; 200 / 4.755 = 42.1 h.

Data-sheet override

Inputs: 20 kW diesel, 50% load, spec-sheet 10 L/h at full load

Result: 5.54 L/h

10 x (0.108 + 0.892 x 0.5) = 5.54.

100 kW diesel flat out in US units

Inputs: 100 kW; 100% load; diesel

Result: 8.08 US gal/h

(0.033 + 0.273) x 100 = 30.6 L/h / 3.785 = 8.08 gal.

An estimate for planning - actual consumption depends on the engine, its condition and the load.

Limitations

  • The diesel coefficients are an example from HOMER's documentation, not a particular engine; real sets differ, so prefer the data-sheet figure.
  • Petrol, LPG and natural-gas figures rest on an assumed 25% efficiency and are rough.
  • Altitude, temperature, engine age and power factor are not modelled.

Where publishers use it

  • Generator rental and sales sites quoting running costs
  • Construction-site and event power planning
  • Telecom, farm and remote-site energy comparisons against solar
  • Power-cut guides for homes and small businesses
  • Facility managers budgeting standby testing

Questions

How much diesel does a generator use per hour?

With the linear fuel curve used here, about 0.306 L per kWh at full load: a 20 kW set uses about 6.1 L an hour flat out and 4.76 L at 75% load. Your set's data sheet gives its own figures at 25/50/75/100% load - enter the full-load one for a better answer.

Why is fuel per kWh worse at low load?

The engine burns part of its fuel just to keep turning. In this model that idle share is about 11% of full-load consumption, so at 25% load each kWh costs much more fuel than at 75%.

How are petrol, LPG and natural gas estimated?

From their energy content - 112,114 Btu per gallon for E10 petrol and 84,250 for propane (US DOE AFDC lower heating values), 1,037 Btu per cubic foot for delivered natural gas (EIA) - and an assumed 25% full-load efficiency. That is a rough figure; a spec-sheet number is better.

How long will my tank last?

Divide the tank by hourly use: 200 L at 4.76 L/h is about 42 hours of continuous running. For LPG cylinders, tanks are usually filled to only about 80%, so use the actual fuel in the tank.

Is it cheaper to run a generator or buy grid power?

Compare the cost per kWh shown with your tariff. At 1.60 per litre a diesel set at 75% load costs about 0.51 per kWh in fuel alone, before maintenance - usually far above grid prices.

How much propane does a generator use?

With the assumed 25% efficiency, about 0.61 litres (0.16 US gal) of liquid propane per kWh at full load: a 20 kW set at 75% load uses roughly 9.5 litres (2.5 US gal) an hour, so a 100 lb cylinder holding about 23.6 US gal would last around 9 hours - check your data sheet.

How much natural gas does a standby generator burn?

About 13 cubic feet (0.37 m3) per kWh at full load on the assumptions here, so a 20 kW standby set at 75% load draws roughly 205 cu ft an hour. Make sure the gas meter and supply pipe are sized for that flow - utilities often need to upgrade the meter.

Sources

  1. Generator fuel curve intercept coefficient - HOMER Energy, HOMER Pro documentation . Fuel curve F = F0 x Ygen + F1 x Pgen; F0 is no-load fuel use divided by rated capacity (L/hr/kW rated).
  2. Generator fuel curve slope - HOMER Energy, HOMER Pro documentation . F1 is the marginal fuel consumption (L/hr/kW output); the documentation's worked example gives F1 = 0.273 and F0 = 0.033.
  3. Fuel Properties Comparison - US Department of Energy, Alternative Fuels Data Center . Lower heating values: gasoline/E10 112,114-116,090 Btu/gal; low-sulfur diesel 128,488 Btu/gal; propane 84,250 Btu/gal.
  4. Frequently asked questions: natural gas units and heat content - US Energy Information Administration . Average heat content of natural gas delivered to end users: about 1,037 Btu per cubic foot.

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A2Z Tools Generator Fuel Calculator
https://a2z.tools/embed/generator-fuel-calculator

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