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How this calculator works
The runtime calculator estimates how long your fuel lasts at four loads: 25%, 50%, 75% and 100%. The headline figure uses 75% load.
- Usable fuel. Tank size × number of tanks × your current fill level × 90%. The 90% allows for fill limits and fuel below the pickup tube.
- Fuel burn. About 0.07 gallons per kWh at full load, times a part-load curve: 25% load uses 31% of full-load fuel, 50% uses 52%, 75% uses 74%. If you pick a generator model, its listed 75% figure is used instead.
- Site adjustments. Planning allowances add about 3.5% per 500 m above 1,000 m (3,281 ft), 3% per 10°C above 40°C (104°F), and 5-10% for older engines.
- Runtime. Usable fuel divided by gallons per hour, with days based on the hours per day you run.
- Checks. It compares your tank with a 96-hour target sized on full-load fuel plus a 133% margin. NFPA 110 §5.5.3 sets a fuel-capacity margin above the class run time. How it applies depends on edition and class; confirm with your AHJ. It also notes when total tank capacity is over 1,320 gallons, where an SPCC plan may be required.
All figures are estimates. Small generators often burn more fuel per kW than the model assumes, so confirm with your spec sheet. Running below about 30% load for long periods can cause wet stacking.
Sources
- NFPA 110, Standard for Emergency and Standby Power Systems
- 40 CFR Part 112, EPA SPCC rule
- ASTM D975, Standard Specification for Diesel Fuel
- Generator manufacturer fuel consumption and derating data for your model