What does the generator calculator work out?
Generator fuel consumption depends mostly on the load: a diesel set burns more fuel per hour at full load and more fuel per kWh at light load. The calculator takes the fuel figures from your set's data sheet, finds the fuel use at your average load, and gives fuel and cost per day and month, liters per kWh and how long the tank lasts.
It also shows the load as a share of the rating against the bands in Caterpillar's white paper on underloading, and compares the fuel issued to a generator with the fuel its run hours should need.
Why check generator load and fuel?
- Site generators run long hours: camps, batching plants, tower cranes and lighting can keep a set running most of the day, so fuel is a large running cost.
- Oversized sets waste fuel: a set running at light load uses more fuel for each kWh it produces.
- Light load harms the engine: Caterpillar warns that running below 30% of rated output for long periods leads to wet stacking and engine damage.
- Fuel issued is easy to lose track of: comparing liters issued with liters expected shows when the log, the meter or the tank needs a look.
What inputs do you need?
| Input | Where to find it | Tip |
|---|---|---|
| Prime rating and power factor | Nameplate, data sheet | Use the prime rating for sets that run as the main power source on site. |
| Average and peak load | Panel meter, kWh meter, load list | A week of meter readings gives a better average than the connected load. |
| Fuel at 25, 50, 75 and 100% load | Manufacturer's data sheet | Take the column for the rating you use, prime or standby. |
| Run hours | Hour meter | Read it at every refuel and at the start and end of each month. |
| Fuel issued | Diesel issue log, bowser meter | Adjust for the change in tank level between the two readings. |
How is fuel and load calculated?
Load share and bands
The bands are from Caterpillar's white paper: standby and prime diesel sets are designed to run between 50 and 85% of nameplate, and light loading below 30% for extended periods harms the engine. Caterpillar recommends at least 30% load for about 30 minutes for every four hours of light-load running.
Fuel at your load
Data sheets list fuel at 25, 50, 75 and 100% load. Between two points the calculator draws a straight line; below 25% it extends the first segment, and it marks that result as an estimate outside the data sheet.
Fuel issued against expected
Sources
- Brian Jabeck, Caterpillar Electric Power, The Impact of Generator Set Underloading, white paper, October 2013. Source: Caterpillar.
- Cummins, C500 D5 diesel generator set data sheet, EMERD-5935-EN, August 2017 (prime 455 kVA, 364 kW; fuel at prime 32.9, 51.6, 71.9 and 94.3 L/h at 25, 50, 75 and 100% load). Source: Cummins data sheet.
- Herc Rentals, How to Calculate Diesel Generator Fuel Consumption, 3 October 2025 (fuel per hour from the manufacturer's chart × run hours). Source: Herc Rentals. Last reviewed 21 October 2026.
Worked example: a 455 kVA set on a construction site
The defaults are an example; replace them with your set and your site.
- 455 kVA × 0.8 = 364 kW. An average load of 200 kW is 55%, inside the 50 to 85% design range; the 300 kW peak is 82%.
- At 55% the fuel use is 51.6 + (55% − 50%) ÷ 25% × (71.9 − 51.6) = 55.6 L/h. Twenty hours a day is 1,112 L a day and 33,369 L in a 30-day month.
- That is 0.278 L per kWh, 7% more per kWh than at full load (94.3 ÷ 364 = 0.259 L per kWh). At USD 0.44 a liter, fuel costs USD 489 a day.
- A 1,000 L tank lasts about 18 hours, so this set needs a refuel every day.
- Over 600 run hours, 36,500 L were issued against 33,369 L expected: 9.4% more, above a 5% allowance. Check the issue log, the hour meter and the tank before drawing conclusions.
- The same set on an 80 kW average load with a 150 kW peak runs at 22%, below the 30% light-load line, and uses about 0.38 L per kWh, 48% more than at full load. A set of 188 to 200 kVA prime would keep that load in the design range and still cover the peak.
How to read your results
- Load under 30%: the set is too large for the load. Move it to a bigger load, share it between areas or replace it with a smaller set.
- Peak above 100%: the set cannot carry all loads at once. Stagger large starts or add capacity.
- Liters per kWh: compare sets on the same unit. The lower the figure, the less fuel for the same power.
- Fuel issued above expected: the gap is a question to answer: a higher load than assumed, a wrong reading, a leak or fuel going elsewhere.
How to cut generator fuel
- Size the set to the load: keep the average load in the 50 to 85% range.
- Switch off when not needed: night and break periods with no load still burn fuel.
- Log every refuel: record liters, hour meter and tank level in the Site Diesel Tank and Bowser Issue Log.
- Service on schedule: follow the manufacturer's service intervals by run hours.
Assumptions and limits
The calculator uses one average load for the period, while real load changes during the day, so actual fuel can differ from the estimate. Data sheet figures are for a set in good condition under test conditions; heat, altitude, wear and fuel quality change them. Below 25% the result is extended beyond the data sheet. It does not size a generator for motor starting, harmonics or site standards; an electrical engineer does that. All defaults are examples, not equipment data.