Load factor
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Generator Fuel Consumption / Runtime / Cost Calculator
Estimate generator fuel burn, runtime, and operating cost in two ways: enter a known vendor burn rate at your expected load, or estimate burn from electrical load, fuel energy content, and your assumed efficiency. Everything runs locally in your browser.
Inputs
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
Direct burn-rate result
Fuel burn rate—
Usable fuel—
Estimated runtime—
Cost per hour—
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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Enter inputs and run the calculator. Results stay in your browser and are not sent anywhere.
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
Planned fuel need: planned fuel = burn rate × planned hours
Estimate mode burn rate: burn rate = electrical load kW / (fuel energy kWh per unit × efficiency)
The default fuel energy values use published U.S. heat-content references converted from Btu per gallon to kWh per gallon. They are useful for estimation only. Real-world generator burn depends on engine tuning, ambient conditions, alternator losses, fuel quality, and partial-load behavior.
When to use each mode
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.
Example
If a diesel generator has a 120-gallon tank, you keep 10% in reserve, and the generator burns 3.2 gal/hr at your operating load, usable fuel is 120 × 0.90 = 108 gal. Runtime is 108 ÷ 3.2 = 33.75 hours. At $4.10/gal, the operating cost is about $13.12/hr.
FAQs
How accurate is estimate mode?
It is only as good as the efficiency assumption. Manufacturer fuel-consumption tables are usually more accurate because they capture the actual engine and alternator behavior.
Why can runtime fall sharply at low or high load?
Real generators are not perfectly linear. Light-load inefficiency, parasitic losses, and high-load enrichment can shift actual burn away from a simple energy-balance estimate.
Should I enter rated kW or expected load kW?
In estimate mode, rated kW is used to show load factor and flag overload. Expected load kW is the number used in the fuel-burn calculation.
Can I use liters instead of gallons?
Yes. Switch the volume unit to liters and the tool converts fuel energy, burn rate, runtime, and price calculations consistently.
Is this suitable for code compliance or life-safety systems?
No. Use manufacturer data, local code requirements, and a qualified engineer for emergency systems and final design decisions.
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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
Fuel energy defaults are based on published U.S. references:
- Diesel: 137,381 Btu/gal
- Gasoline: 120,214 Btu/gal
- Propane: 91,452 Btu/gal
These values are converted using 1 kWh = 3,412 Btu. For a different fuel blend or site-specific specification, use the custom fuel option.
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.