Generator Fuel Calculator

Calculate how much fuel your generator uses per hour, how long it will run on available fuel, and how much fuel you need for a power outage. Supports all common fuel types.

Generator Fuel Calculator

Generator Size (watts)

Load Percentage (%)

Fuel Type

Fuel Available (gallons) (optional)

Outage Duration (days) (optional)

Fuel Price (per gal) (optional)

How do you calculate domestic fuel consumption on a generator?

Generator fuel consumption is primarily determined by three variables: generator size (in kilowatts), the percentage of rated load the generator is currently running, and the fuel type. Most manufacturers publish fuel consumption data at 25%, 50%, and full load — the figures in this calculator are based on industry-standard consumption rates at each load level.

The fundamental formula: Fuel per Hour = Base Rate (per kW) × Generator kW × Load Fraction. For a 5 kW petrol generator at 50% load: 0.5 gal/kW/hr × 5 kW × 0.5 = 1.25 gallons per hour.

Which generator fuel is cheapest to run?

Petrol

The most common generator fuel in residential applications. A typical portable petrol generator consumes 0.5–0.6 gallons per kilowatt per hour at full load. Petrol has a shelf life of 3–6 months without stabiliser (12 months with stabiliser) — critical for emergency preparedness stockpiling. Ethanol-blended fuels (E10, E15) have shorter shelf lives and can cause fuel system corrosion in generators stored for extended periods.

Diesel

Diesel generators are 20–30% more fuel-efficient than petrol equivalents. A 5 kW diesel generator at 50% load uses approximately 0.9–1.0 gallons per hour versus 1.25 for petrol. Diesel stores for up to 12 months with fuel stabiliser and is generally more energy-dense. Most standby generators above 20 kW use diesel. Running diesel generators below 30% rated load for extended periods can cause "wet stacking" — incomplete combustion that deposits unburned fuel in the exhaust system.

Propane (LP Gas)

Propane generators are popular for their indefinite fuel storage life — propane does not degrade with age like liquid fuels. However, propane has lower energy density than petrol or diesel, requiring approximately 20–25% more fuel volume for equivalent output. Propane generators are well-suited for homes with existing LP supply (rural properties, areas with unreliable fuel supply chains) and are popular for standby whole-house generator systems.

Natural Gas

Natural gas generators connect directly to the utility supply line, eliminating fuel storage entirely. This makes them the most convenient option for extended outages — as long as the gas supply remains intact. Natural gas generators are primarily used as permanent standby systems. Consumption is measured in cubic feet (CF) or cubic meters (m³) per hour, and consumption costs vary significantly by utility rate and region.

How much fuel do you need for an extended outage?

The key insight for emergency preparedness planning is that continuous operation is rarely necessary. Running a generator 6–8 hours per day for essential functions (refrigerator cycling, phone charging, medical equipment, lighting) consumes far less fuel than continuous operation. For a 3-day outage at 8 hours/day with a 5,000-watt generator at 50% load using petrol: 1.25 gal/hr × 8 hrs × 3 days = 30 gallons — a realistic stockpile for most households.

Refrigerator: 150–200 watts average running load; can be cycled on/off every few hours
Sump pump: 750–1,250 watts when running (intermittent)
Window AC unit: 1,000–1,500 watts (significant load — avoid running continuously)
LED lighting (6 fixtures): 60–120 watts total
Phone/laptop charging: 100–200 watts total
Total essential load estimate: 600–800 watts average (approximately 15–20% of a 5 kW generator)

Worked example

A rural property owner sizes fuel storage for a 6 kW diesel generator that runs the house essentials during outages, typically at about half load for twelve hours a day.

Inputs

Generator rating: 6 kW
Applied load: 50%
Fuel type: Diesel (≈0.30 L/kWh)
Run time: 12 h/day for 3 days
Fuel price: $2.439/L

1Electrical output6 kW × 50% = 3.0 kW
2Hourly burn3.0 kW × 0.30 L/kWh = 0.90 L/h
3Fuel for 36 hours0.90 L/h × 36 h = 32.4 L
4Cost32.4 L × $2.439 = $79

0.9 litres an hour, 32.4 litres and $79 for a three-day outage at twelve hours a day.

Storage sizing should assume worse than this. Light loads are disproportionately inefficient, so if the load drops to a quarter the consumption per kilowatt-hour rises rather than halving. Two 20-litre jerrycans is the honest minimum for a three-day plan, and stored diesel needs rotating before it degrades.

Frequently Asked Questions

How much fuel does a generator use per hour?

A typical 5,000-watt (5 kW) petrol generator at 50% load consumes approximately 1.25 gallons (4.7 litres) of petrol per hour. At full load (100%), consumption rises to around 2.5 gallons (9.5 litres) per hour. Diesel generators are 20–25% more fuel-efficient. This calculator adjusts for both generator size and load percentage.

How long will a generator run on 20 litres of petrol?

A 5,000-watt generator at 50% load (2,500 watts) running on petrol consumes approximately 1.25 gallons per hour, so 5 gallons provides about 4 hours of runtime. At 25% load (1,250 watts), consumption drops to approximately 0.625 gallons per hour, extending runtime to 8 hours. Enter your specific figures in the calculator for an exact estimate.

Is diesel or petrol more efficient for generators?

Diesel generators are typically 20–30% more fuel-efficient than equivalent petrol generators. A 5 kW diesel generator at 50% load uses approximately 0.9–1.0 gallons per hour versus 1.25 gallons for petrol. Diesel also stores better for emergency preparedness — up to 12 months with fuel stabiliser versus 3–6 months for petrol. However, diesel generators typically cost 30–50% more upfront.

How much natural gas does a generator use per hour?

Natural gas generators consume approximately 9–12 cubic feet (cf) of natural gas per kilowatt per hour at full load. A 10 kW natural gas generator at 50% load (5 kW effective output) uses approximately 45–60 cf/hour. Natural gas home generators are connected directly to the utility supply line, eliminating fuel storage concerns during extended outages.

How much fuel do I need for a 3-day power outage?

For a 5,000-watt petrol generator at 50% load running continuously for 3 days (72 hours), you need approximately 90 gallons (340 litres) of fuel — which is impractical for most households. In practice, running a generator for 6–8 hours per day (for essential appliances, refrigerator cycling, and phone charging) requires 8–10 gallons for a 3-day outage. Use this calculator with a partial load to model realistic consumption.

Generator Fuel Calculator