Generator Fuel Runtime & Cost Estimator

direct answer

To calculate generator fuel consumption, multiply the generator's rated kilowatts by the load percentage and the fuel's specific fuel consumption rate: fuel per hour = kW × load % × SFC. Divide the tank size by that burn rate to get total runtime, then multiply the burn rate by your fuel price for the cost per hour.

inputs

no submit button

Generator size

Rated output — copy it from the spec plate.

Fuel type

SFC 0.16 gallons per kW per hour.

Load percentage

75%

Engines burn the least fuel per kilowatt between 50% and 75% load — idling wastes fuel.

Fuel tank size

gal

Switching units converts the tank size and price, so the results never change.

Current fuel price

per gal

Any currency — no symbol assumed.

results — live

updates as you type

Fuel burn at this load

at 75% load (3.75 kW)

0.60 gal/h

3.75 kW × 0.16 gal/kWh = 0.60 gal/h

Full-tank runtime: 8.3h ≈ 8 h 20 min on 5 gal
Cost to fill the tank: 17.50 5 gal × 3.50 per gal
Cost to run per hour: 2.10 0.60 gal/h × 3.50 per gal

in plain words

Costs use the same currency unit you typed for the fuel price.

methodology

How This Calculator Works (The Formula)

Every number this tool returns comes from one constant and four equations. The constant is the generator's specific fuel consumption (SFC) — the amount of fuel the engine burns for every kilowatt it delivers, for every hour it runs.

Real power, if you entered kVA: kW = kVA × 0.8. Generators are sold as either kVA (apparent power) or kW (real power), and a 0.8 power factor is the standard conversion for a resistive mixed load.
Fuel per hour: fuel/h = kW × load % × SFC. Only the kilowatts actually being drawn burn fuel, so the load percentage is applied before the SFC constant.
Total runtime: hours = tank size ÷ fuel per hour.
Cost to fill the tank: tank size × price per unit.
Cost to run per hour: fuel per hour × price per unit.

SFC Constants Used by This Calculator

Fuel	SFC constant	Unit
Gasoline	0.16	gallons per kW per hour
Diesel	0.07	gallons per kW per hour
Propane / LPG	0.19	gallons per kW per hour
Natural Gas	11	cubic feet per kW per hour

Those constants translate to a thermal-to-electric efficiency of roughly 17% for gasoline, 36% for diesel, 20% for propane and 30% for natural gas, which matches published performance for standby and portable gensets. Diesel wins on fuel volume because a compression-ignition engine extracts more work from each gallon.

Worked Example

A 5 kW gasoline generator running at 75% load with a 5-gallon tank at 3.50 per gallon:

Loaded power: 5 kW × 0.75 = 3.75 kW
Fuel per hour: 3.75 × 0.16 = 0.60 gal/h
Runtime: 5 ÷ 0.60 = 8.3 hours (8 h 20 min)
Cost per hour: 0.60 × 3.50 = 2.10
Cost to fill: 5 × 3.50 = 17.50

Set the inputs above to those values and the live readout reproduces every figure — that is the whole audit trail.

Runtime Reference (5 kW Gasoline Generator, 5-Gallon Tank)

Load	Power drawn	Fuel per hour	Runtime
25%	1.25 kW	0.20 gal	25 h 0 min
50%	2.50 kW	0.40 gal	12 h 30 min
75%	3.75 kW	0.60 gal	8 h 20 min
100%	5.00 kW	0.80 gal	6 h 15 min

Why Real-World Numbers Vary

The SFC constants are typical figures at full load with clean fuel and a healthy engine. Expect the burn rate to move with altitude (roughly 3% of output lost per 1,000 feet, which is why a mountain-site generator produces less and works harder), air temperature, engine age, eco-mode and load quality. Running below about 30% load is the classic mistake: the engine still spins at speed, so fuel per kilowatt delivered climbs sharply. Check the spec plate on your unit and, when it matters, meter a tank over one hour.

Related Guides

How Much Gas Does a 5000-Watt Generator Use Per Hour? — the most-searched size, broken down by load level.
Diesel vs Gasoline vs Propane: The Cheapest Generator Fuel — cost per hour for all four fuels, and the break-even price that decides.
How Long Will a Generator Run on a Full Tank? — runtime charts by tank, load and size, plus the 2-hour rule.

faq

Frequently Asked Questions

The three questions people actually search for, answered with the same formula the calculator uses.

How much gas does a 5000-watt generator use per hour?

A 5000-watt (5 kW) gasoline generator burns about 0.8 gallons per hour at full load, because 5 kW × 0.16 gal/kW/h = 0.8 gal/h. At the more common 75% load it uses 0.6 gallons per hour, and a 5-gallon tank then lasts about 8.3 hours.

How long will a generator run on a full tank?

Divide the tank size by fuel consumption per hour. A 5 kW gasoline generator at 75% load burns 0.6 gallons per hour, so a 5-gallon tank lasts 5 ÷ 0.6 = 8.3 hours. At full load the same tank lasts 6.25 hours; at 25% load it lasts 25 hours.

Which fuel is cheapest to run: gasoline, diesel, propane, or natural gas?

Diesel uses the least fuel per kilowatt-hour — 0.07 gallons per kW-hour against 0.16 for gasoline, 0.19 for propane and 11 cubic feet for natural gas. Diesel usually costs least per hour unless it sells for more than about 2.3 times the price of gasoline; piped natural gas is typically cheapest of all.