What This Calculator Does
This generator runtime calculator tells you how long a full tank keeps your backup generator running, and what that power costs.
Most off-grid homes keep a generator for the long grey spells when the panels can't keep up. It's the spare tyre of the system: you hope you rarely need it, but you want to know how far it gets you. On a windy site, a small wind turbine beside the panels can shorten those grey spells too.
How To Use This Calculator
- In Generator rated power, type the continuous kW on the nameplate, not the bigger peak number. Only a kVA rating? Multiply it by the power factor (cos φ) on the plate. No power factor listed? Multiply by 0.8.
- In Fuel type, pick diesel, gasoline, propane or natural gas. The fuel price box jumps to a matching starting value.
- In Fuel on site, type how much fuel you store: litres for diesel and gasoline, kilograms for propane. Natural gas comes through a pipe, so this box disappears.
- In Load level, type how hard the generator works, as a percentage of its rating. Not sure? Leave it at 50.
- In Average daily home consumption, type what your home uses per day in kWh. A modest off-grid home uses about 5 to 12 kWh. The energy use calculator works yours out.
- In Fuel cost, type what you pay per litre, kilogram or cubic metre.
- Read the result card: how long the fuel lasts running non-stop, and the power it delivers. The Breakdown shows fuel burn per hour, cost per hour, cost per kWh, fuel per kWh, the kWh the fuel can make, the generator hours a day your home needs, and days of backup with a battery bank.
The Formula, In Plain Words
First the calculator works out the power output: rated kW times your load level.
Then it looks up how much fuel one kWh takes at that load. The numbers come from real spec sheets, measured at 25, 50, 75 and 100% load. In between, the calculator draws a straight line.
For diesel it uses the average of three Cummins generators of 12 to 22 kW, each at its standby and prime rating. That is 0.61 litres per kWh at 25% load, 0.40 at 50%, 0.34 at 75% and 0.32 at full load. Gasoline comes from three Honda inverter generators, propane and natural gas from two Kohler home standby sets.
Power times fuel per kWh gives the burn rate. The fuel on site divided by the burn rate is your non-stop runtime. The fuel divided by fuel per kWh is the electricity it can make. Divide that by your daily use and you get days of backup. Your daily use divided by the power output tells you how many hours a day the generator has to run.
A Worked Example
Take the starting values: a 5 kW diesel at 50% load, 25 litres of fuel, a home using 12 kWh a day, and diesel at €2.46 per litre, the Dutch pump average on 19 October 2026.
The output is 5 × 0.5 = 2.5 kW. At 0.40 litres per kWh it burns 1.0 litre an hour.
Running non-stop, the fuel lasts 25 hours, at €2.46 an hour or €0.98 per kWh. In that time it makes 62.5 kWh.
Your home needs 12 kWh a day, so the generator runs 12 ÷ 2.5 = 4.8 hours a day. A battery bank stores what the house doesn't use right away and covers the rest of the day. That spreads the 25 hours over about 5.2 days. Without a battery the generator has to run all day at a tiny load, burns far more per kWh, and the fuel lasts far fewer days.
Run the same diesel at full load and each kWh takes only 0.32 litres. The 25 litres then last 15.6 hours but hold 78 kWh, about 6.5 days.
Switch to gasoline at 50% load and the same 25 litres last about 12.7 hours, at about €1.89 per kWh with petrol at €2.39.
What To Watch Out For
A generator uses the least fuel per kWh near full load. At light load it still spins fast for little power.
A diesel running too lightly can also clog with unburned fuel. Engineers call this "wet stacking". Caterpillar warns about long runs below 30% load.
The diesel numbers come from 12 to 22 kW generators. A small 3 to 6 kW diesel usually burns more per kWh, so the calculator warns you when your size falls outside the measured range. Treat the result as a best case, and use your own spec sheet if it lists fuel use at 25, 50, 75 and 100% load.
Days of backup assume a battery bank whose charger can take the generator's output, and the generator running only the hours a day your home needs. If the generator can't make your daily kWh even running 24 hours, the calculator says so instead of showing days. Generators belong outdoors, at least 20 feet (6 m) from doors, windows and vents, because the exhaust contains deadly carbon monoxide. Read about off-grid energy sources to see where a generator fits.
Hard Words
| Term | What it means |
|---|---|
| kW | Kilowatt: the useful power a generator delivers. |
| kVA | Kilovolt-amps, another size on nameplates. kW is kVA times the power factor, often 0.8 and sometimes 1.0: check the plate. |
| Continuous rating | The power a generator can deliver for hours, not just a short burst. |
| Load level | How hard the generator works, as a percentage of its rating. |
| Burn rate | How much fuel the generator uses per hour. |
| Power factor | The share of kVA that becomes useful power (kW). Written as cos φ on many nameplates. |
| Wet stacking | Unburned fuel and soot building up in a diesel that runs too lightly for too long. |
Common Questions
How long will a generator run on a full tank?
Divide the fuel you store by the hourly burn rate. In this calculator, a 5 kW diesel at 50% load burns about 1.0 litre an hour, so 25 litres last about 25 hours. Running harder burns fuel faster per hour, and gasoline engines burn more than diesel for the same power.
How much fuel does a generator use per kWh?
Cummins spec sheets for 12 to 22 kW diesels show about 0.61 litres per kWh at 25% load, falling to 0.32 at full load. Honda's gasoline inverter generators use about 0.89 litres per kWh at quarter load and 0.58 at full load. Your own spec sheet is more accurate than these averages.
What is the best load level for a generator?
For fuel, high load is best: the spec sheets show the lowest fuel per kWh at 75 to 100%. Caterpillar warns that long runs below 30% load cause wet stacking in diesels, a build-up of unburned fuel and soot. Sizing the generator close to your real needs keeps it in its efficient range.
What does generator power cost per kWh?
Multiply the fuel price by the fuel used per kWh. With diesel at €2.46 a litre and 0.40 litres per kWh at half load, each kWh costs about €0.98. That is far more than grid power or solar, which is why off-grid homes use a generator as a backup, not as their main source.
How many days of backup does a generator give?
Divide the electricity your stored fuel can make by your home's daily use. In this calculator, 25 litres of diesel at half load make about 62.5 kWh. For a home using 12 kWh a day, that is about 5.2 days, but only with a battery bank: the generator runs about 4.8 hours a day and the battery covers the rest. Running non-stop, the same fuel lasts 25 hours.