Generator Fuel Consumption & Runtime Planning (Screening)
Introduction #
When this guide fits: After Generator Size Calculator screening, you need order-of-magnitude fuel and tank runtime for OPEX or logistics—not a manufacturer performance map.
When it is not suitable: Emissions compliance, parallel gen sets, or contracted maintenance SLAs—use OEM fuel curves and project specs.
Start from facility load on Factory Load and backup path on Power Calculator hub. For UPS bridge during start/transfer, use Generator + UPS Calculator and ATS Transfer Guide.
Screening formulas (diesel, planning only) #
Average fuel rate (L/h or gal/h):
Fuel rate ≈ (kW_load × specific_fuel_consumption) ÷ (fuel_density × generator_efficiency)
Practical shortcut for budgeting:
| Load (% of rated kW) | Typical diesel use (indicative) |
|---|---|
| 25% | ~0.05–0.08 L/kWh |
| 50% | ~0.07–0.10 L/kWh |
| 75% | ~0.09–0.12 L/kWh |
| 100% | ~0.11–0.15 L/kWh |
Values vary by engine tier, altitude, and maintenance—replace with OEM chart before procurement.
Runtime from tank:
Runtime (h) ≈ Usable_tank_volume (L) ÷ Fuel_rate (L/h)
Use 80–90% usable tank volume unless code requires higher reserve.
Worked example #
Given:
- Critical load 200 kW (from factory load roll-up)
- Generator screened 250 kVA @ PF 0.8 → 200 kW electrical capability at full load
- Operating point 75% of rated kW → 150 kW average during outage
- Planning 0.10 L/kWh at that load point
- Diesel tank 1,000 L, 85% usable → 850 L
Fuel rate:
- 150 kW × 0.10 L/kWh = 15 L/h (screening)
Runtime:
- 850 ÷ 15 ≈ 57 h continuous at 150 kW (idealized—verify with OEM)
Sizing check: If policy requires 72 h autonomy, increase tank or reduce assumed load—do not downsize generator kVA without motor-start review.
Runtime vs UPS bridge (different questions) #
| Question | Tool / guide |
|---|---|
| How long until gen is online? | ATS Transfer Guide |
| How long UPS carries load? | UPS Runtime / Generator + UPS |
| How long diesel lasts at kW? | This guide (screening) |
Load factor and fuel #
Low load factor (see Load Factor Guide) means average kW is below peak—fuel budgeting should use expected average kW during outage, not nameplate sum of all connected equipment.
Common mistakes #
- Using connected kW instead of critical/gen-set kW.
- 100% tank volume without unusable bottom/safety margin.
- Ignoring altitude/temperature derating on engine output and fuel rate.
- No test run—commission with load bank per maintenance plan.
Next steps #
- Size kVA: Generator Size Calculator
- Motor inrush: Motor Starting & Protection
- Feeder OCPD: Breaker Size → Cable Size
- Full backup workflow: Power Calculator hub
FAQ #
Is there a generator runtime calculator on CalcPanel?
No dedicated runtime tool. Use this guide for screening fuel and hours; size kVA with Generator Size Calculator.
How do I estimate fuel consumption per hour?
Multiply average kW during outage by a L/kWh factor from the table above or OEM data. Adjust for efficiency and site conditions.
Does generator sizing include fuel tank size?
No. kVA sizing covers electrical capacity; tank volume is a separate logistics and code decision.
How does UPS affect generator fuel planning?
UPS covers transfer gap only; generator fuel must support steady kW after transfer—plan both with Generator + UPS.
When is load bank testing required?
Many facilities test annually or after major engine work—document in maintenance plan; not covered by web calculators.