How much fuel will my backup generator burn, and how long will it run?

The formula: it is the load that drinks, not the badge on the generator

A generator's fuel use follows the energy it actually delivers. The calculator multiplies four things: rated size in kW, load factor (the share of that rating you are really using), hours of runtime, and a burn rate of 0.078 gallons per kWh for diesel at 25 °C and sea level. It then adds a 10% safety margin, because planning an outage to the last cup of fuel is how freezers thaw.

That load factor is where estimates usually go wrong. A 20 kW standby unit rarely carries 20 kW of house. The calculator's own presets put a standard home's essential loads at 35% — fridge, lights, furnace blower, a few outlets — not the 75–100% people picture when they size a fuel store.

Worked examples, straight from the calculator's presets

These are three of the presets built into the fuel calculator, with its temperature and altitude adjustments applied:

• Halve the load and you halve the fuel — in the calculator's tests, a unit at 50% load burns exactly half of the same unit at full load over the same hours.
• Runtime is linear too: the hourly burn does not change, only the total. If you know gallons per hour, tank runtime is just usable gallons ÷ that rate.
• Hourly burn follows the power actually delivered, not the badge: the 40 kW unit at 25% load delivers 10 kW, so at 0.86 gal/h it burns less per hour than the 30 kW winter-storm unit delivering 13.5 kW (1.17 gal/h) — but more than the 20 kW standard-home unit delivering 7 kW (0.60 gal/h). Right-sizing the load list beats obsessing over the rating.

Cold, altitude, and biodiesel each take a cut

The base rate assumes a mild day at sea level. The calculator adjusts it three ways, and the adjustments multiply together:

• Cold: below 0 °C, consumption rises 0.2% for every degree — a −10 °C night costs about 2% more fuel, and −30 °C is the calculator's tested extreme.
• Heat: above 30 °C, it rises 0.1% per degree.
• Altitude: +3% per 1,000 m. The high-altitude preset (2,000 m) carries a 6% penalty before anything else is counted.
• Biodiesel: +7% consumption versus diesel in the calculator, and priced at $4.20/gal against diesel's $3.80 — it costs more per gallon and burns more gallons.

So how long will a tank last?

Take the fuel you actually have, divide by the hourly burn for your real load, and stop there — that is the honest runtime. For the standard-home example above, 10 gallons on hand ÷ 0.60 gal/h is about 16 hours of essential-loads cover. The trap is dividing by the wrong rate. Use the full-load burn rate for a generator that will actually loaf along at a third load and the answer comes out far shorter than the runtime you really have — safe, but it sends you out to buy and store fuel you did not need. Use a light-load rate for a house that will actually run near full load and the error flips: the tank empties before your estimate says it should, mid-outage. The rate has to match the load you will really carry.