Diesel vs Gasoline vs Propane: The Cheapest Generator Fuel, Actually Calculated

The constant nobody publishes

Every generator burns fuel at a rate set by its engine, not by its badge. Engineers call it specific fuel consumption (SFC) — the mass or volume of fuel needed to produce one kilowatt-hour of electricity. For the four generator fuels, the widely used field values are:

That efficiency column is the whole story. A compression-ignition diesel engine squeezes roughly twice as much electricity out of a gallon as a gasoline engine does — 36% against 17%. Propane pays for its clean burn and infinite shelf life with the thirstiest engine of the group. Natural gas sits in between, but nobody metering gas from a pipe ever counts it by the gallon, which is why it usually feels free.

Cost per hour, side by side

Take a 5 kW generator at 75% load — 3.75 kW drawn, the typical real-world figure — and these illustrative prices: gasoline 3.50/gal, diesel 3.80/gal, propane 2.75/gal, natural gas 1.50 per 100 cubic feet.

Over a single overnight outage the gap looks trivial. Run the same four machines for a full week without stopping and the table reads 103.95 on natural gas, 167.58 on diesel and 352.80 on gasoline. Choose the fuel before the storm, not during it.

The one break-even number that decides it

Prices move; the ratios do not. Compare two fuels by dividing their SFC constants — that ratio is the price gap required to flip the winner:

• Diesel vs gasoline: 0.16 ÷ 0.07 = 2.29. Diesel stays cheaper until it costs 2.3× gasoline per gallon. A gap that wide is almost unheard of at retail.
• Propane vs gasoline: 0.16 ÷ 0.19 = 0.84. Propane is cheaper per hour only below about 84% of the gasoline price — 2.95/gal when gasoline is 3.50. Worth checking at your supplier, but rarely true.
• Natural gas vs gasoline: gasoline at 3.50/gal means 0.56 per kilowatt-hour. Natural gas wins while it stays below 0.56 ÷ 11 = 0.051 per cubic foot — i.e. under about 5.10 per 100 cubic feet. Mains gas is normally 1.00–3.00.

So the practical ranking for most buyers is natural gas, diesel, gasoline, propane — and the interesting part is that it only takes one local price shock to swap the middle two.

Price per hour is not the whole cost

The hourly burn rate is what the fuel costs. What it costs to own depends on how you will actually use the machine:

• Storage life: gasoline goes stale in 3–6 months without stabilizer; diesel lasts 6–12 months; propane and natural gas do not degrade at all. For a generator that sits idle for a year, propane wins on the fuel you do not have to rotate.
• Refuelling at 2 a.m.: diesel is less flammable than gasoline and safer to handle, propane comes from a valve, and natural gas never runs out while the mains is up.
• Maintenance: diesel gensets cost more up front but routinely outlast gasoline units. Propane burns cleanest, with almost no carbon buildup on the valves.
• Availability during a crisis: petrol stations need power to pump. Gas lines need pressure, which usually survives. Diesel can be stored in your own tank indefinitely.
• Cold starts: diesel is harder to start below freezing without glow plugs or a block heater; gasoline and propane start readily.

Rank those against your situation and the cheapest fuel to run may not be the cheapest fuel to own.

Do the math with your own prices

The formula is cost per hour = kW drawn × SFC × price per unit. Two multiplications, and you have a decision. If you would rather drag sliders than multiply, the Generator Fuel Runtime & Cost Estimator takes your generator size, fuel type, tank and local price, and returns fuel per hour, cost per hour, cost to fill and full-tank runtime as you type.