Generator Fuel Calculator

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

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.