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; at 0.835 kg/L that is 0.275 L/kWh, or ~0.21 L/h per rated kW when running at 75% of rating. Vendors publish a fuel curve in their datasheets — this calculator uses the cross-vendor median, calibrated against Cummins, Caterpillar, Generac, and Kohler datasheets for 100–800 kVA sets. Engine size matters: small sets under 50 kVA burn 10–20 % more per kW, and large electronically-injected sets over 1 MVA burn 5–10 % less, so the calculator flags both. For mission-critical sizing, always substitute your specific genset's curve.
NFPA 110: Class, not a flat 24-hour rule
NFPA 110 does not set a single fuel duration. It defines the emergency power supply by Class — the minimum time it must run at rated load without refuelling — and leaves the Class to the authority having jurisdiction and the application: Class 2 (2 h) is the usual life-safety minimum for egress lighting and fire pumps, Class 6 for many hospitals and telecom sites, Class 48 or 96 for data centres and critical operations, or “Class X” where the owner specifies the hours. The fuel supply is then sized at a minimum of 133 % of the Class duration's consumption (NFPA 110 §5.5) to cover fuel-quality losses and the unusable volume at the bottom of the tank. The calculator applies exactly that rule: pick the Class, and it reports whether your usable fuel clears 1.33 × hours × consumption at the connected load.
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, and refuses to report an autonomy figure when the connected load exceeds the derated capacity — that is an overload, not a runtime.