Calculate generator fuel efficiency from a fuel log by dividing delivered electrical energy in kWh by fuel consumed during the same interval. Compare matching operating conditions, because the lowest gallons per hour can still produce fewer useful kWh per gallon.
Open the Generator Fuel Efficiency Calculator.
Decide which efficiency measure you want
For a liquid-fuel generator, kWh per gallon = delivered kWh ÷ gallons consumed. A larger result means more electrical energy was delivered per gallon within the stated measurement boundary.
The reciprocal measure is gallons per kWh = gallons consumed ÷ delivered kWh. Here, a smaller result means less fuel per unit of electricity. Both describe the same data; choose one primary measure and label it consistently.
Neither is a percentage thermal-efficiency calculation. To calculate a percentage, you would also need the fuel’s energy content on a defined heating-value basis. A kWh/gal figure alone does not identify the fraction of chemical energy converted to electricity.
Keep power and energy distinct. The EIA explains kW as a power unit and kWh as energy delivered over time in its electricity measurement guide. Rated generator kW is not the electrical energy produced every hour unless the machine actually delivers that power for the hour.
Build a log with matching boundaries
Record the start time, stop time, electrical energy-meter readings, fuel inventory or reliable consumption readings, fuel additions, and significant changes in operating conditions. Include the generator model and the fuel unit used.
The electrical and fuel measurements must cover the same period. Measuring fuel across a full day and energy across only the busiest six hours produces an inflated efficiency figure. The reverse mismatch can make a healthy generator look inefficient.
Choose whether warm-up and unloaded operation belong in the analysis. A whole-outage fuel-efficiency figure normally includes them because that fuel was consumed to provide the service. A stabilized operating-point comparison may exclude them deliberately. Both can be useful if their boundaries are explicit.
Do not infer fuel burned from deliveries alone. A delivery can replenish inventory for later use. For an appropriate tank-accounting method, consumed fuel is beginning inventory plus additions minus ending inventory, with all quantities on a consistent basis.
Use installed instrumentation or the manufacturer’s approved fuel-accounting method. Do not open a fuel system or improvise plumbing merely to obtain a measurement.
An illustrative eight-hour fuel-log calculation
Suppose a generator’s energy meter rises from 12,450 to 12,486 kWh during an eight-hour operating period. The delivered energy is 36 kWh. A reliable fuel record shows 4.8 US gallons consumed over that same period.
Efficiency is 36 ÷ 4.8 = 7.50 kWh/US gal. Fuel intensity is 4.8 ÷ 36 = 0.1333 US gal/kWh. Average real output is 36 ÷ 8 = 4.5 kW, and average fuel rate is 4.8 ÷ 8 = 0.60 gal/h.
| Logged or calculated item | Value | Why it matters |
|---|---|---|
| Operating time | 8 h | Shared measurement interval |
| Electrical energy delivered | 36 kWh | Useful-output numerator |
| Fuel consumed | 4.8 US gal | Matching fuel denominator |
| Average output | 4.5 kW | Describes the operating point |
| Average fuel rate | 0.60 gal/h | Useful for hourly planning |
| Fuel efficiency | 7.50 kWh/gal | Energy delivered per gallon |
If the generator is rated 10 kW on the applicable fuel and rating basis, its average output during this interval is 45% of that rating. That does not prove it ran steadily at 45%; the load could have varied considerably while producing the same energy total.
A real energy-meter total is preferable to a guess that several appliances operated at their nameplate input for the entire eight hours. Cycling loads can make that guess inaccurate.
What if the log contains only load and runtime?
If you have reliable average real kW rather than an energy total, estimate energy as average kW multiplied by hours. A measured average of 6 kW over five hours gives 30 kWh.
When the load changes, split the period into intervals. For example, 3 kW for two hours and 9 kW for one hour produces 3 × 2 + 9 × 1 = 15 kWh across three hours. The time-weighted average is 5 kW, not the simple average of 3 and 9.
If the available reading is amperes or kVA, do not assume it equals real kW. Voltage, phase, and power factor determine the conversion, and changing power factor complicates the average. Use an appropriate real-power or energy measurement where practical.
An approximate load estimate should be reported as such. A precisely displayed calculator result does not improve the precision of an uncertain input. Preserve a sensible range when appliance operation or the average load is poorly known.
Compare low and higher load without confusing hourly fuel use
Consider two illustrative stabilized operating points for the same machine and fuel. At a lighter load it supplies 2 kW and consumes 0.40 gal/h. At a higher load it supplies 6 kW and consumes 0.75 gal/h.
The first point delivers 2 ÷ 0.40 = 5 kWh/gal. The second delivers 6 ÷ 0.75 = 8 kWh/gal. The lighter load burns less fuel each hour, while the higher load produces more electrical energy from each gallon.
This is a comparison of example measurements, not a universal claim that a particular percentage load is always optimal. Use the actual generator’s data and operating limits. Manufacturer fuel tables can show how consumption varies across specified load points; Cummins provides such entries in its C25 D6 sheet.
Do not add unnecessary electrical demand just to improve a ratio. An operating plan should serve useful loads, meet the equipment’s requirements, and preserve the needed reserve. A smaller or differently configured power source may be a separate comparison worth evaluating with actual data.
Use the corrected calculator
The corrected fuel-productivity tool now uses average electrical kW divided by actual liquid-fuel US gal/h, without fuel-type or age/condition multipliers. Those conditions are already represented by the measurements. Natural-gas volume is not offered as undefined gallons. Keep kWh/US gal distinct from percentage fuel-energy efficiency and compare the same measurement periods.
Combine several sessions with weighted totals
To combine logs, add delivered kWh and consumed gallons first, then divide. Do not average session efficiency ratios unless the fuel weighting makes that appropriate.
Suppose session A delivers 20 kWh using 4 gal, or 5 kWh/gal. Session B delivers 80 kWh using 8 gal, or 10 kWh/gal. Together they deliver 100 kWh from 12 gal, which is 8.33 kWh/gal.
The simple average of 5 and 10 is 7.5, which gives the small session too much influence. A report based on total energy and fuel avoids that error and can be traced back to its records.
Group logs sensibly. Separate different generators, fuels, measurement methods, or materially different conditions unless a combined service figure is the intended result. If several generators feed a shared bus, decide how electrical output and fuel are attributed before comparing individual efficiency.
Look for measurement error before diagnosing deterioration
A change from 8 to 7 kWh/gal may reflect a different load schedule, more unloaded time, an inventory-reading error, or a changed measurement boundary. It is not by itself evidence of a mechanical fault.
Small fuel quantities are especially sensitive to reading error. If the log reports 2.0 gal but the uncertainty is ±0.2 gal, the denominator may be off by 10%. Reporting several decimal places would imply confidence the measurements do not support.
Compare repeated records under reasonably similar conditions and retain notes on ambient conditions, load profile, and instrumentation. If efficiency shifts persist, give the service provider the original log rather than only a final ratio. The time, output, fuel, and operating notes help distinguish a changed job from a changed machine.
Keep gross fuel flow separate from fuel burned
Some fuel systems circulate more fuel through the engine supply path than the engine actually consumes. A supply-flow reading alone may therefore be unsuitable as the consumed-fuel denominator when fuel returns to the tank.
Use a measurement arrangement intended to report actual consumption or an appropriate inventory balance. Do not subtract unrelated meter readings without checking their units, accuracy, and time boundaries. If the log identifies only fuel supplied rather than net fuel consumed, label the limitation and obtain the appropriate equipment information before interpreting a low kWh-per-gallon result as poor engine efficiency.
References
- EIA: measuring electrical power and energy.
- Cummins C25 D6 generator load-point data.
- Cummins T-030 generator application manual.