Where the consumption figure comes from
Brake-specific fuel consumption (BSFC) is the diesel engine's consumption per kWh of mechanical output. A typical mid-size diesel sits around 230 g/kWh at 75% load, which translates to ~0.18 L/h per rated kW at 75% load. Vendors publish a fuel curve in their datasheets — this calculator uses the cross-vendor median, calibrated against Cummins, Caterpillar, Generac, and Kohler datasheets. For mission-critical sizing, always substitute your specific genset's curve.
NFPA 110: the 24-hour rule
For life-safety standby generators (hospitals, high-rise residential, transit, telecom), NFPA 110 Level 1 mandates 24 hours of on-tank fuel at full rated load. The calculator flags installations that fall short of this threshold. Non-life-safety applications commonly target 8–12 hours; mission-critical data centres often go to 72 hours plus a standing refuel contract.
Natural gas vs diesel
NG gensets are popular in regions with a strong gas grid (much of North America, parts of Europe and East Asia) because they don't need on-site fuel storage and emit less particulate. Trade-off: a gas-grid outage knocks out the genset too. The autonomy figure for NG is constrained by the gas-meter capacity, not a tank, so the user enters expected runtime gas in m³.
Derating at altitude and high ambient
ISO 8528-1 Annex C derates naturally-aspirated diesels by roughly 1% per 100 m above 1000 m and 2% per 5 °C above 25 °C ambient. Turbocharged engines derate more gently — confirm with the genset vendor. The calculator applies both deratings to the rated output before computing the load fraction.