In short. Generator loading is the running load as a share of the rating. Too high leaves no room for steps and growth; too low risks wet-stacking and poor efficiency. A good design chooses a loading band for normal operation and keeps real headroom for starts, steps and the failure of another unit.
Loading terms
Spare capacity (kW) = Rating − Running load
Use kW for the engine and kVA for the alternator, and keep the lower margin of the two as your limit.
Why not run at the limit
- No headroom for motor starts and load steps.
- Lower margin against derating, ageing and fuel quality.
- More engine stress and shorter life.
- No room for growth.
Why not run very light
- Wet-stacking: unburnt fuel and carbon in the exhaust at low load, which can foul injectors and exhaust systems.
- Poor fuel efficiency.
- Possible shortened engine life without periodic higher-load runs.
Many sets benefit from regular running at higher load. A load bank can provide that where site load is small. See load bank testing.
A loading band by duty
| Duty | Typical thinking |
|---|---|
| Emergency standby | Moderate loading; short runs; larger headroom for starts |
| Prime power | A band that allows for variable load and long hours; follow the manufacturer’s prime rating |
| Continuous | Steady loading at a level the manufacturer recommends |
| Parallel plant | Run only as many units as needed to keep each in a healthy band |
See prime power and standby power. Ratings and permitted loading depend on the manufacturer’s definitions.
Reading a load profile
- Get a load profile over the period, with minimum, average and peak.
- Compare peak with rating for headroom.
- Compare minimum with the generator’s preferred minimum load.
- Check how long the load stays in each band.
Worked example (illustrative)
A 500 kW generator serves a site with load between 80 kW and 360 kW. Peak loading is 72 per cent, leaving 140 kW of headroom, good for steps. Minimum loading is 16 per cent, which is too light for long running. Options: split into two 250 kW units that run as needed so each stays in a healthier band; or add a controlled dummy load; or schedule regular loaded runs. The decision is about the profile, not the peak alone.
Multi-generator effect
In a parallel plant, automatic start and stop of units keeps the running units well loaded. See operating combinations.
Healthy loading band
A diesel generator prefers moderate loading. Very light running causes incomplete combustion, carbon deposits and wet stacking; running at the limit leaves nothing for a surge.
| Loading | Typical consequence |
|---|---|
| Below about 30 percent | Wet stacking risk if sustained, poor fuel economy |
| 40 to 80 percent | Healthy band for most diesel sets |
| Above 90 percent | No margin for surges, higher stress |
Confirm band limits with the engine maker.
Spare capacity worked example (illustrative)
A 500 kW set carries 320 kW. Loading = 64 percent. Spare = 180 kW. If the largest step to be added is a 90 kW motor with a starting factor of 3 on real power, the instantaneous demand is about 320 + 270 = 590 kW, which exceeds 500 kW. The check passes only if the voltage and frequency dips are acceptable, which needs the transient guide, or if the starting method is changed.
Load schedule format
- Phase-by-phase table of steady loads.
- Steps in the order they connect on transfer.
- Peak and minimum cases.
Improving poor loading
Where a set runs lightly most of the time, consider multiple smaller sets, a load bank on periodic tests or battery support. See load bank testing and load-step screening guide.
Common mistakes
- Sizing from peak only and ignoring light-load operation.
- Assuming ratings for standby apply to continuous use.
- Using no headroom for growth.
What to send us
Send your load profile data. Send your application details. See also capacity requirement.
Questions engineers ask
What loading is best for a diesel generator?
Many manufacturers favour a moderate to high loading band, avoiding long periods at very light load. Follow your engine maker’s guidance.
What is wet-stacking?
Unburnt fuel and soot accumulating in the exhaust and engine from running at low load.
How do I deal with light load?
Use smaller or fewer units, a load bank for periodic runs, or hybrid arrangements with storage.
Does spare capacity include the failed-unit case?
For redundant plants, yes. The remaining units must carry the load when one is out.
Figures are illustrative. Follow the manufacturer’s rating definitions and loading guidance.