In short. A generator’s rating applies at defined reference conditions. Real sites are often hotter, higher or more restricted, so the usable output is lower. Apply derating for altitude, temperature and installation, and use the manufacturer’s curves for the final figure.
Why derating happens
- Altitude: thinner air gives the engine less oxygen, so it makes less power.
- High ambient temperature: hot air is less dense, and cooling margin shrinks; the alternator also runs hotter.
- Humidity: reduces combustion air oxygen content slightly.
- Enclosure and ventilation: restricted airflow raises intake and engine bay temperatures.
- Fuel quality and exhaust or intake restrictions: reduce output.
- Alternator thermal limit: temperature rise class and ambient define the continuous rating.
Illustrative effect
| Condition | Typical effect |
|---|---|
| Altitude above a threshold, often around 1,000 m for naturally aspirated and higher for turbocharged engines | Percentage loss per additional 100 m |
| Ambient above reference, often 25 to 40 °C | Percentage loss per extra 5 °C or 10 °C |
| Enclosed set with restricted ventilation | Additional derating or cooling upgrade |
The actual thresholds and percentages are specific to each engine and alternator. Use the manufacturer’s data sheets for the model concerned.
How to apply it
- Find the site’s highest ambient design temperature and altitude.
- Read the factors from the manufacturer’s derating curves.
- Multiply them, then compare the derated output with the required load.
- If it is short, choose a larger set, improve ventilation or reduce load.
Worked example (illustrative)
A set rated 500 kW at reference conditions is to run on a rooftop plant room where the design ambient is 45 °C. Suppose the manufacturer gives a temperature factor of 0.93 at that temperature, with an installation factor of 0.97 for the enclosure. Derated output = 500 × 0.93 × 0.97 ≈ 451 kW. If the site load with growth is 460 kW, the set is short. The designer either selects a larger unit, adds cooling air, or lowers the load. The numbers here are examples; only the manufacturer’s curves apply.
Cooling and ventilation
Check that the room or enclosure supplies enough cool air for combustion and radiator, and discharges hot air without recirculation. See generator room ventilation. Poor ventilation can produce overheating at loads that the nameplate suggests are fine.
Related pages
Why generators lose capacity
Engine power depends on air density and cooling. Hot, thin or restricted air reduces the oxygen available for combustion and the heat the cooling system can reject.
| Factor | Typical effect (indicative; use maker curves) |
|---|---|
| Ambient temperature above reference (often 25 or 40 °C) | About 1 to 2 percent per 5 °C above the engine’s rated limit |
| Altitude above about 1,000 m (naturally aspirated) or the turbo limit | About 3 to 4 percent per 300 m |
| Humidity | Small effect |
| Restricted intake or recirculating hot air | Can exceed the other effects |
| Alternator temperature rise | Class H machines at 40 °C versus higher at site |
Worked example (illustrative)
A 500 kW set rated at 40 °C on a site with an effective intake of 48 °C because of room recirculation. If the maker’s curve gives 1.5 percent per 5 °C beyond 40 °C, the loss is about 2.4 percent, leaving 488 kW. Altitude of Hyderabad (about 540 m) is below typical derating thresholds, so no altitude loss. The practical derating is small, but recirculation in a poorly ventilated room can be far worse.
Using the guide
- Establish worst-case ambient and altitude.
- Read the maker’s derating curve for the engine and alternator.
- Apply the larger of engine or alternator loss, not both combined.
- Check room ventilation.
See ambient temperature derating, altitude derating and room ventilation.
Common mistakes
- Using the rating without conditions.
- Using the average ambient instead of the design maximum.
- Ignoring recirculation of hot air in a plant room.
- Applying generic factors when the manufacturer gives specific curves.
What to send us
Send the site altitude, design temperature, installation type and the generator model if known. Send your application details.
Questions engineers ask
Does derating apply to the alternator too?
Yes. Alternator ratings are for a stated ambient and temperature rise. Hotter conditions reduce the permitted output.
Is derating needed in a hot city?
If the design ambient exceeds the reference temperature, yes. Check the manufacturer’s data for the set.
Can I avoid derating by improving cooling?
Better ventilation helps with installation effects, but intake air temperature and altitude effects remain.
Where do I find the factors?
In the manufacturer’s technical data for the engine and alternator, which give curves for temperature and altitude.
Figures are illustrative. Use the manufacturer’s derating data for design.