What is a generator log sheet?
A generator log sheet is the daily record of a set's running: hour meter readings, load, energy produced, fuel added and tank levels, with the operator's name. On construction sites, camps and batching plants it is often the only record of how many hours a set ran and how much diesel went into it.
This log goes one step further. It compares the fuel each set used with the fuel its data sheet says it should use at the load it carried, so a gap stands out the same day instead of at month end.
How to use the generator log
- Register each unit with its prime rating, power factor and the fuel use at 25, 50, 75 and 100% load from its data sheet.
- Read the meters at the start and end of each day or shift: hour meter, kWh meter if fitted, and the tank level in liters.
- Record every refuel in Fuel added, with the liters from the bowser meter or the issue log.
- Record the load in kW from the panel meter when there is no kWh meter. For a compressor, type a load % if you know it.
- Review the flags each day and write the reason in Notes.
- Review the monthly summary and the service status with the site manager.
How does the log calculate fuel and load?
Caterpillar's white paper says standby and prime diesel sets are designed to run between 50 and 85% of nameplate, and that light loading below 30% for extended periods harms the engine; both bands are on Settings. The allowed fuel difference and the service warning are your choice.
Sources: Brian Jabeck, The Impact of Generator Set Underloading, Caterpillar, October 2013 (Source: Caterpillar); Cummins, C500 D5 data sheet, EMERD-5935-EN, August 2017 (Source: Cummins data sheet). Last reviewed 21 October 2026.
Worked example: two 455 kVA sets and a compressor
Both example generators use the Cummins C500 D5 data sheet: 455 kVA prime, 364 kW, and 32.9, 51.6, 71.9 and 94.3 L/h at 25, 50, 75 and 100% load.
- GN-01, 4 October: 4,000 kWh in 20 hours is 200 kW, a 55% load. The data sheet gives 55.6 L/h, so 1,112 L were expected. The tank went from 790 L, plus 1,400 L added, to 830 L: 1,360 L used, 22.3% above expected, over the 5% allowance.
- GN-02 carries about 70 kW, a 19% load, every day, so each row shows Light load. In October it uses 0.412 L per kWh against 0.287 L per kWh for GN-01, 44% more fuel for each kWh.
- GN-02, 5 October: 480 L were added but the end-of-day tank reading is missing, so fuel used is blank and the row is flagged.
- AC-01 has no fuel curve yet, so its liters and hours are logged (21.5 L/h on average) without a difference.
- Service: GN-01 is 240 hours past its last service on an example 250-hour interval, so it shows Service due soon; GN-02 is at 275.5 hours and shows Service overdue.
Common mistakes with generator logs
- Writing fuel added but no tank levels. Without both readings you cannot tell fuel used from fuel stored.
- Reading the meter only at refuels. Daily readings show which day the gap happened.
- Using the standby column of the data sheet for a set that runs as the main supply. Use the column for the rating it runs at.
- Ignoring light load. A large set on a small load wastes fuel and, below 30% for long periods, harms the engine.
- One log for several sets. Each set needs its own rows, or the comparison with its data sheet is lost.
What the template does not do
The log works with the readings you enter; it does not read the generator or the tank. Data sheet figures are for a set in good condition, and below 25% load the expected fuel is extended beyond the data sheet. A difference is a question to answer, not proof of theft or a fault. Service intervals come from the manufacturer's manual. To estimate fuel and size a set before you hire it, use the Generator Fuel Consumption & Load Calculator; to track issues from a site tank or bowser, use the Site Diesel Tank and Bowser Issue Log.