Generator Fuel Calculator
Calculate how long your generator will run on a full tank, or how much fuel you need to store for a given number of days.
Based on generator wattage and load.
How long does a tank last?
Generator runtime depends on three things: the actual electrical load (not the rated capacity), the fuel-to-energy efficiency of the engine, and tank size. Manufacturers usually publish runtime at 50% load, which most owners never read carefully and then complain when their generator runs out faster than expected.
The math
A typical petrol generator burns about 0.33 litres per kWh of electrical output at 50% load. The number rises slightly above 75% load (heat losses) and falls slightly below 25% load (idle inefficiency).
litres_per_hour = (rated_watts × load_percent ÷ 100) ÷ 1000 × 0.33
runtime_on_tank = tank_litres ÷ litres_per_hour
A 3500 W generator running at 50% load (1750 W) burns about 0.58 L/hour. A standard 15 L tank lasts about 26 hours of continuous running.
Petrol vs diesel vs propane
| Fuel | L/kWh | Storage life | Storage hazard |
|---|---|---|---|
| Petrol (gasoline) | 0.33 | 6 to 12 months treated | Volatile, fumes |
| Diesel | 0.27 | 12 to 24 months treated | Less volatile |
| Propane | 0.50 (L equivalent) | indefinite | Pressurized |
| Natural gas | n/a | utility supplied | none if pipe survives |
For long-term emergency storage, propane is the only fuel that does not degrade. Petrol with stabiliser keeps for about a year; without it, six months and it gums injectors. Diesel keeps longer but grows microbial slime in the tank if water gets in.
Practical sizing
Plan generator runtime at 8 hours per day, not 24. Running a generator continuously is loud, eats fuel fast, and shortens engine life. Eight hours covers refrigeration cycles, evening lighting, charging phones and tools, and a few hours of well pump if you are on rural water.
For 30 days at 8 hours per day with 1500 W of essential loads, you need about 30 L of fuel per day, or 900 L stored. That is impractical for most households, which is why most preppers settle on 7 to 14 day supplies and accept that anything longer means restoring grid power.
Safety
Never run a generator indoors or in an attached garage. Carbon monoxide poisoning from generators kills more people each year than the storms that knocked out power. 6 metres / 20 feet from any opening, downwind from the house, sheltered from rain, with a working CO alarm inside.