Generator Fuel Cost Calculator

About the Generator Fuel Cost Calculator

Generator fuel cost is a major ongoing expense, especially during extended power outages or for off-grid operations. The cost depends on three factors: the fuel consumption rate (gallons per hour), the fuel price, and how many hours the generator runs. Fuel consumption increases with load — a generator at 50% load uses significantly less fuel than at full load.

Diesel generators are the most fuel-efficient but diesel fuel is often more expensive. Natural gas generators have lower fuel costs where piped gas is available. Propane generators offer portability and long storage life but higher fuel cost per kWh. Gasoline generators are cheapest to buy but most expensive to run and the fuel has a short shelf life.

This calculator estimates hourly, daily, and monthly fuel costs for any generator. Enter the fuel consumption rate from your generator's specifications (or measure it) and the current fuel price. Use it for outage planning, off-grid budgeting, and comparing generator types by operating cost.

When This Page Helps

Fuel is the largest operating cost of a generator. This calculator helps you budget for extended outages, compare fuel types, and optimize your generator's load level for fuel efficiency. Regular monitoring of this value helps energy teams detect usage anomalies early and address equipment malfunctions or operational issues before they drive utility costs higher.

How to Use the Inputs

1. Find your generator's fuel consumption rate (from the manual or spec sheet).
2. Enter the consumption rate in gallons per hour.
3. Enter the current fuel price per gallon.
4. Enter the expected hours of operation per day.
5. View hourly, daily, and monthly fuel costs.
6. Compare costs at different load levels to find the economical operating point.

Example Calculation

Tips & Best Practices

• Run generators at 50–75% load for optimal fuel efficiency.
• Running at light load (<30%) causes wet-stacking in diesels and wastes fuel.
• Store diesel fuel with stabilizer — it degrades in 6–12 months without treatment.
• Natural gas from utility lines provides essentially unlimited fuel during outages.
• Propane doesn't degrade and can be stored indefinitely — ideal for standby generators.
• Monitor fuel levels and plan refueling before the tank drops below 25%.
• Dual-fuel generators (propane + gasoline) offer fuel flexibility.

Fuel Consumption by Generator Size

Portable 3.5 kW (gasoline): 0.4 gal/hr at 50% load. Portable 7.5 kW: 0.7 gal/hr. Standby 12 kW (propane): 1.2 gal/hr. Standby 20 kW (natural gas): 204 ft³/hr. Commercial 100 kW (diesel): 7 gal/hr. Commercial 500 kW (diesel): 28 gal/hr. All values at approximately 75% load.

Comparing Fuel Types

Diesel: 138,700 BTU/gallon. Highest energy density. Propane: 91,500 BTU/gallon. Clean-burning. Natural gas: 1,030 BTU/ft³. Cheapest where piped. Gasoline: 120,000 BTU/gallon. Most accessible but shortest shelf life (3–6 months without stabilizer).

Total Cost of Ownership

Fuel is 60–80% of a generator's lifetime cost. Maintenance (oil, filters, coolant) adds 10–20%. Purchase price is only 10–20% of lifetime cost for frequently-used generators. When choosing generator type, focus on fuel cost per kWh rather than purchase price.

Frequently Asked Questions

• How much fuel does a home generator use?

  A typical 10–20 kW home standby generator at 50% load: propane: 1–2 gal/hr, natural gas: 100–200 ft³/hr. A 7 kW portable: gasoline: 0.5–1.0 gal/hr. Larger generators use proportionally more fuel.

• Which fuel type is cheapest to run?

  Per kWh generated: natural gas is cheapest ($0.05–$0.10/kWh), diesel is mid-range ($0.10–$0.20/kWh), propane is similar to diesel ($0.12–$0.22/kWh), gasoline is most expensive ($0.15–$0.30/kWh). Prices vary by region.

• Does load level affect fuel consumption?

  Yes, significantly. A generator at 25% load uses about 50–60% of the fuel it uses at full load — but generates only 25% of the power. The most fuel-efficient operating point is typically 50–75% of rated load.

• How do I reduce generator fuel costs?

  Reduce connected loads (shed non-essentials). Run at 50–75% load for efficiency. Use a load management system. Maintain the generator regularly (clean air filters, fresh oil). Consider a more efficient unit if running frequently.

• How much diesel does a commercial generator use?

  A rule of thumb for diesel generators: 7 gallons per hour per 100 kW at full load. A 200 kW generator uses about 14 gal/hr at full load. At 75% load, consumption drops to about 10.5 gal/hr.

• How long will a propane tank last on a generator?

  A 500-gallon propane tank (filled to 80% = 400 usable gallons) powering a 20 kW generator at 50% load (~1.5 gal/hr) lasts about 267 hours or 11 days of continuous operation. A 250-gallon tank lasts about 5.5 days.