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Generator fuel consumption and run time

Enter the rating, how many hours the set spends in each load band and the tank. The page works out average fuel use, fuel per kWh, how long the tank lasts and the tank size for the required run time.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Preliminary estimate
Generator set
Load profile

Hours in each band over a typical operating period (a day, an outage or a shift).

Fuel curve

Example values for a 500 kVA prime-rated set. Replace them with your unit’s data sheet.

Tank and run time
Cost
—Run time on the tank, with the load profile
—

Fuel use and specific consumption versus load

Lines: fuel per hour from the curve in use and from the published linear model. Points: fuel per kWh.

Tap the chart to see the values.

Tank run time at constant load

Hours of running on the usable tank volume if the set stayed at one load. The dashed line is the required run time.

Tap the chart to see the values.

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

P_rated = kVA × PF; linear model: fuel (L/h) = a × P_rated + b × P_output (a = 0.08145; b = 0.246 L/(h·kW))
Maker’s curve: linear interpolation between 25%, 50%, 75% and 100%, extrapolated along the end segments
Average fuel = Σ h_i·C(f_i) ÷ Σ h_i; specific consumption = Σ h_i·C(f_i) ÷ Σ h_i·f_i·P_rated
Run time = V_tank × usable share ÷ average fuel; tank for t hours = fuel × t ÷ usable share

Assumptions and limits

  • Diesel density 0.84 kg/L (7.0 lb/gal) and emission factor 2.68 kg CO₂ per liter (22.4 lb/gal).
  • The published linear model tends to overestimate fuel for sets above 100 kVA; it is a conservative reference when no data sheet is available.
  • Fuel cost only: maintenance, lube oil, DEF and depreciation are excluded. Altitude and temperature above ISO 3046-1 conditions reduce available power.
  • Emergency system codes such as NFPA 110 size fuel for the class duration at full demand load; use the full-load run time in that case.

Standards and technical documents

  • ISO 8528-1: Reciprocating internal combustion engine driven alternating current generating sets — Application, ratings and performance
  • ISO 3046-1: Reciprocating internal combustion engines — Performance — Declarations of power, fuel and lubricating oil consumptions
  • Skarstein, Ø.; Uhlen, K. (1989): Design considerations with respect to long-term diesel saving in wind/diesel plants. Wind Engineering 13(2)
  • HOMER Energy: HOMER Pro user manual, generator fuel curve
  • NFPA 110: Standard for Emergency and Standby Power Systems

U.S. regulations

  • NFPA 110: Standard for Emergency and Standby Power Systems (fuel supply by class)
  • NFPA 30: Flammable and Combustible Liquids Code; NFPA 37: stationary combustion engines
  • EPA 40 CFR Part 60 Subpart IIII: stationary compression ignition engines; 40 CFR Part 63 Subpart ZZZZ
  • EPA 40 CFR Part 112: Spill Prevention, Control and Countermeasure (SPCC)

Frequently asked questions

How much fuel does a 500 kVA generator burn?

At full load (400 kW) a modern set burns about 28 gal/h; at 50%, about 15 gal/h. The published linear model gives 34.6 gal/h at full load, a more conservative figure.

How do I work out tank run time?

Divide the usable volume by average fuel use. A 500 gal tank at 90% usable and 18.2 gal/h average lasts about 25 h.

Why does fuel per kWh rise at light load?

Part of the fuel is burned even with no load: friction, pumps and the fan. At 25% load a 400 kW set uses about 0.09 gal/kWh, against 0.07 gal/kWh at 75%.

What tank do I need for 24 hours?

Multiply average fuel use by 24 and divide by the usable share: 18.2 gal/h × 24 ÷ 0.9 ≈ 485 gal. If NFPA 110 full-load sizing applies, use 28 gal/h: about 747 gal.