There is one thing that often confuses facility operators: generators that are rarely loaded actually seem to consume more fuel. The liters per kWh increase, even though the engine is running lightly. This is not a meter reading error, but rather a characteristic of diesel engines that has long been documented by manufacturers.
Benchmark Figures
Diesel engines are designed to operate at medium to high loads. Common guidelines state that the minimum load should be 30 percent of the nameplate capacity, while the ideal operating range is around 60 to 75 percent.
The ISO 8528 standard makes this limit explicit. In that standard, there are four ratings: Emergency Standby Power with total operation not exceeding 200 hours per year, Prime Power for unlimited operating hours, Limited Time Running Prime with a limit of 500 hours per year, and Continuous Power for constant loads. Both ESP and PRP limit the average load factor over 24 hours to 70 percent of their rating.
There is an interesting detail in how the standard calculates. When calculating the average power from a variable load profile, power below 30 percent of the rating is still counted as 30 percent. The standard itself already considers operation below this threshold as an abnormal condition that should not be maintained.
What Happens Below the Threshold
Low loads cause pressure and temperature in the combustion chamber to not be high enough. Diesel does not burn completely, leaving behind soot and raw fuel in the exhaust gas channels. The symptoms are typical: sticky black liquid dripping at the exhaust joint, the smell of raw diesel, dark smoke, and carbon deposits on the turbocharger turbine side. This phenomenon is called wet stacking, or exhaust slobber in its visual form.
The chain of consequences moves slowly but in one direction. Raw fuel can dissolve the lubricant layer on the cylinder walls, triggering cylinder glazing, reducing the effectiveness of the piston sealing ring, and causing diesel to seep into the crankcase. Oil viscosity decreases, lubrication ability diminishes, and bearing wear accelerates. The unusual fuel consumption is merely the earliest symptom visible in the monthly reports.
As a rough guideline, continuous operation below 30 percent capacity carries a high risk of carbon deposit accumulation. The range of 30 to 50 percent needs to be monitored. Above 50 percent is generally safer, provided that the exact figures still follow the manufacturer's manual.
One common misconception is to consider normal coolant temperature as evidence of normal combustion. The two measure different things. Coolant reflects the heat dissipation of the cooling system, not the temperature of the combustion chamber. Wet stacking can still occur even if the cooling panel appears safe.
Proving It, Not Guessing
The actual load profile needs to be read from data, not from estimates. Load testing with a load bank provides a controlled artificial load so that performance can be verified without risking critical facility equipment. The testing is usually done in stages, for example at 25, 50, 75, and 100 percent, although that order is not fixed and must follow the machine's characteristics and agreed procedures. ISO 8528-5:2025 establishes performance criteria for the generator's response to load changes, including limits on voltage and frequency deviations.
What is recorded is not just kW. Voltage per phase, current, frequency, power factor, engine speed, coolant temperature, oil pressure and temperature, and ambient temperature are also recorded, then compared with the limits set by the manufacturer. Without that comparison, the test results are not very meaningful.
When the Load is Indeed Small
If the reading results show that the facility load is indeed well below the threshold, the issue is no longer maintenance but capacity selection. Generators that far exceed actual needs will continue to be in the danger zone, no matter how many times the oil is changed.
The consequences are also felt when designing hybrid systems. The feasibility study for adding PLTS (solar power plants) to a system still supported by diesel depends on how much load is available to bear. If the existing load is already below the diesel minimum threshold, adding solar panels does not automatically improve the situation, as the limitation lies at the operating point of the machine, not on the availability of solar energy. This characteristic is what makes optimization studies conducted first, before determining the capacity of the PLTS. This topic is also discussed in the Hybrid PLTS Engineering & Simulation class, and if you prefer to see the explanation directly, there is a short clip.
Three things worth checking before concluding that your generator is wasteful: what percentage of the average load is maintained throughout the day according to the controller data, how long the unit has operated below 30 percent, and when the last time the unit was load tested.
Sources
- PT Interjaya Suryamegah, "What Are the Effects of Underloading a Generator?". interjaya.com
- Powerline Service Genset (PT Ducotti Dieselindo Persada), "Generator Exhaust Leaking Oil? Complete Solution to Wet Stacking Issues", July 7, 2026. servicegensetpowerline.com
- Powerline Indonesia, "Load Bank Testing of Generators: How to Verify Performance and Capacity", August 28, 2026. powerline.co.id
- SG Power Plus, "Power Ratings of Diesel Generator as per ISO 8528", summary of ESP, PRP, LTP, and COP ratings along with the 70 percent load factor limits. sgpowerplus.com