Load factor
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Generator Fuel Consumption Calculator: Runtime and Cost
Calculate generator fuel use per hour, runtime from tank size or available fuel, and fuel cost for diesel, gasoline, propane, and natural gas. Start with a generator-size lookup, or use a known manufacturer burn rate for the most accurate result.
Quick answer
Runtime equals usable fuel divided by fuel burn rate. If 108 gallons are usable and the generator burns 3.2 gal/hr, runtime is about 33.8 hours.
Fuel cost per hour equals burn rate times fuel price. At 3.2 gal/hr and $4.10/gal, fuel costs about $13.12 per hour.
Inputs
gal
$/gal
hours
Fast planning estimate for common search tasks such as fuel use by generator size and load. The direct burn-rate mode is more accurate when you have manufacturer data.
gal/hr
Best for vendor spec sheets or measured field data. Enter the burn rate that matches your actual operating load.
Estimate mode uses fuel energy content and efficiency: burn rate = load kW / (fuel kWh per unit × efficiency). For custom fuel, enter energy content per gallon and the tool converts to liters when needed.
Results
Size/load lookup result
Fuel burn rate—
Usable fuel—
Estimated runtime—
Cost per hour—
Estimated cost per day—
Fuel needed for planned run—
Cost for planned run—
Estimated full usable-tank cost
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Fuel energy assumption
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Refuel interval
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Planned runtime coverage
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| Duration | Fuel needed | Estimated cost | Entered supply covers? |
|---|---|---|---|
| 8 hr | — | — | — |
| 12 hr | — | — | — |
| 24 hr | — | — | — |
| 48 hr | — | — | — |
| 72 hr | — | — | — |
Enter inputs and run the calculator. Results stay in your browser and are not sent anywhere.
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Formulas and assumptions
Usable fuel: usable fuel = tank size × (1 - reserve %)
Runtime: runtime hours = usable fuel / burn rate
Cost per hour: cost/hr = burn rate × price per unit
Cost per day: daily cost = cost/hr × 24
Planned fuel need: planned fuel = burn rate × planned hours
Lookup mode burn rate: burn rate = generator kW × interpolated fuel rate at load %
Estimate mode burn rate: burn rate = electrical load kW / (fuel energy kWh per unit × efficiency)
The lookup tables are planning curves, not manufacturer fuel maps. Real-world generator burn depends on engine tuning, ambient conditions, alternator losses, fuel quality, and partial-load behavior.
Fuel consumption charts
Use these tables for quick lookup answers before entering a custom scenario. The diesel chart estimates gallons per hour by generator size and load.
| Diesel generator size | 25% load | 50% load | 75% load | 100% load |
|---|---|---|---|---|
| 20 kW | 0.52 gal/hr | 0.86 gal/hr | 1.24 gal/hr | 1.62 gal/hr |
| 60 kW | 1.56 gal/hr | 2.58 gal/hr | 3.72 gal/hr | 4.86 gal/hr |
| 100 kW | 2.60 gal/hr | 4.30 gal/hr | 6.20 gal/hr | 8.10 gal/hr |
| 500 kW | 13.00 gal/hr | 21.50 gal/hr | 31.00 gal/hr | 40.50 gal/hr |
Natural gas standby generators are often planned in cubic feet per hour or therms. The table below estimates cubic feet per hour.
| Natural gas generator size | 25% load | 50% load | 75% load | 100% load |
|---|---|---|---|---|
| 20 kW | 72 ft³/hr | 126 ft³/hr | 180 ft³/hr | 240 ft³/hr |
| 60 kW | 216 ft³/hr | 378 ft³/hr | 540 ft³/hr | 720 ft³/hr |
| 100 kW | 360 ft³/hr | 630 ft³/hr | 900 ft³/hr | 1,200 ft³/hr |
| 500 kW | 1,800 ft³/hr | 3,150 ft³/hr | 4,500 ft³/hr | 6,000 ft³/hr |
When to use each mode
Size/load lookup mode is the default because many users need an immediate answer from generator size, fuel type, and load percentage. It interpolates a built-in planning curve and is appropriate for early outage planning.
Direct burn-rate mode is the better option whenever you have a manufacturer fuel curve or measured site data. If the spec sheet says your generator burns 3.2 gal/hr at about 70% load, use that number directly. That bypasses several simplifying assumptions and produces the most realistic runtime and cost estimate.
Estimate-from-load mode is useful earlier in planning, when you know expected electrical load but do not yet have the exact fuel map. The tool converts fuel heat content into kWh per unit, applies your efficiency assumption, and estimates the burn required to produce the electrical load. This is suitable for budget conversations and rough capacity checks, not final emergency-power design.
Reserve fuel matters operationally. Many teams do not plan to use the final 5% to 20% of a tank, either to protect pickup conditions, preserve contingency margin, or align with refueling procedures. This calculator treats reserve as unusable for runtime so the result stays conservative.
Accuracy levels
- Best: manufacturer fuel curve or measured burn rate at the actual load, entered in direct burn-rate mode.
- Good planning estimate: size/load lookup mode using generator kW, fuel type, and load percentage.
- Rough engineering estimate: estimate-from-load mode using heat content and an assumed electrical efficiency.
Use lookup or estimate results for budgeting and outage planning. Use manufacturer data, site test data, code requirements, and qualified engineering review for final emergency-power decisions.
Worked examples
Portable gasoline generator
A 7 kW gasoline generator at 50% load uses about 0.39 gal/hr. A 24-hour run needs about 9.2 gal.
With an 8-gallon tank and 10% reserve, runtime is about 18.7 hr. At $3.80/gal, cost is about $1.46/hr or $35/day.
20 kW propane home standby
At 50% load, a 20 kW propane generator uses about 1.42 gal/hr. A 24-hour run needs about 34 gal.
With 90 usable gallons from a 100-gallon supply, runtime is about 63 hr. At $2.70/gal, cost is about $3.83/hr or $92/day.
60 kW diesel generator
At 50% load, a 60 kW diesel generator uses about 2.58 gal/hr. A 24-hour run needs about 62 gal.
With a 120-gallon tank and 10% reserve, runtime is about 42 hr. At $4.10/gal, cost is about $10.58/hr or $254/day.
100 kW diesel generator
At 75% load, a 100 kW diesel generator uses about 6.20 gal/hr. A 24-hour run needs about 149 gal.
With a 250-gallon tank and 10% reserve, runtime is about 36 hr. At $4.10/gal, cost is about $25.42/hr or $610/day.
Natural gas standby generator
A 22 kW natural gas standby generator at 50% load uses about 139 ft³/hr. A 24-hour run needs about 3,326 ft³, or about 33 therms.
At $1.80/therm, cost is about $2.59/hr or $62/day. Runtime usually depends on gas utility service instead of an on-site tank.
FAQs
How do you calculate generator runtime?
Runtime equals usable fuel divided by fuel burn rate. Usable fuel is tank size multiplied by the percentage you are willing to consume before reserve.
How much diesel does a generator use per hour?
Diesel use depends on generator size and load. As planning estimates, a 20 kW diesel generator may use about 0.6 to 1.6 gal/hr from 25% to 100% load, while a 100 kW unit may use about 2.6 to 8.1 gal/hr.
How long will a generator run on one gallon of fuel?
Runtime per gallon is one divided by the burn rate. A generator burning 0.5 gal/hr runs about 2 hours per gallon, while a generator burning 4 gal/hr runs about 15 minutes per gallon.
How do you estimate fuel burn from electrical load?
Estimated fuel burn is load in kW divided by fuel energy in kWh per unit and divided by efficiency. Manufacturer fuel curves are still the best source when available.
What does load percentage mean for a generator?
Load percentage is the electrical load divided by the generator rated output. A 60 kW generator carrying 30 kW is at 50% load.
Should I use lookup, direct burn-rate, or estimate-from-load mode?
Use direct mode whenever you have a manufacturer or measured burn rate. Use lookup mode for a quick planning estimate from generator size and load. Use estimate mode only when you need a rough engineering estimate from fuel heat content and efficiency.
Can this calculator estimate natural gas generator fuel use?
Yes. Natural gas can be estimated in cubic feet, cubic meters, or therms, with cost per hour calculated from the selected price unit.
Does this calculator account for reserve fuel?
Yes. Reserve fuel reduces the usable tank volume so the runtime result reflects the amount of fuel you plan to keep in reserve.
Why do manufacturer fuel charts differ from calculator estimates?
Manufacturer charts reflect a specific engine, alternator, calibration, test condition, and partial-load behavior. Generic lookup curves and heat-content math are planning estimates and cannot capture all of those details.
Are the results suitable for emergency or life-safety design?
No. This tool is for planning and budgeting. Final emergency-power decisions should use manufacturer data, code requirements, and qualified engineering review.
Generator planning notes
Fuel maps beat generic rules
Spec-sheet gallons-per-hour data at 25%, 50%, 75%, and 100% load usually outperforms any simplified estimate.
Best input
Reserve changes usable runtime
A 10% reserve on a 500-gallon tank removes 50 gallons from the runtime calculation immediately.
Operational margin
Cost per hour helps budget quickly
Hourly fuel cost is often the fastest way to compare standby testing, demand-response events, and outage scenarios.
Budgeting
Partial-load efficiency varies
Actual burn may not scale cleanly with load, especially for lightly loaded or oversized generators.
Model limit
Refueling plans matter as much as tank size
A long theoretical runtime is less useful if deliveries, site access, and transfer procedures cannot support the outage scenario you are planning for.
Operations
Reference assumptions
Last reviewed: June 29, 2026.
Fuel energy defaults are based on U.S. Energy Information Administration heat-content references and rounded generator-planning assumptions:
- Diesel: 137,381 Btu/gal, from EIA petroleum heat-content tables.
- Gasoline: 120,214 Btu/gal, from EIA petroleum heat-content tables.
- Propane: 91,452 Btu/gal, from EIA liquefied petroleum gas heat-content tables.
- Natural gas: 1,037 Btu/ft³ as a representative U.S. heat-content assumption. Actual delivered gas varies by region and utility.
- Conversion constant:
1 kWh = 3,412 Btu, rounded for calculator display.
The lookup curves are generic planning curves built from common generator chart patterns and heat-content scaling. They are included to answer size-and-load lookup questions quickly, not to replace model-specific fuel maps. For a different fuel blend, site-specific specification, compressed-gas billing basis, or tested equipment curve, use the direct burn-rate mode.
Useful public references include the EIA Monthly Energy Review, EIA energy conversion FAQs, and NIST weights and measures resources.
Disclaimer
This calculator is for planning, estimation, and budgeting only. It does not replace manufacturer fuel-consumption data, code review, or engineering sign-off. Verify final runtime, refueling strategy, ventilation, emissions, and emergency-power requirements with the actual generator documentation and qualified professionals.