A generator that runs perfectly at a coastal warehouse can trip on overload the day it reaches a mountain site, with nothing mechanically wrong. The reason is derating. A diesel engine produces its nameplate power only under the reference conditions it was rated at, typically around sea level and 25 °C. Thinner air and higher temperatures both reduce real output. If you buy equipment for high-altitude work, such as mines, farms, telecom sites or construction in mountain regions, derating is the most important number in your sizing calculation.
What does derating mean?
Derating is the reduction in the power an engine can deliver when it runs outside the conditions it was rated in, mainly altitude, ambient temperature and humidity. It is not a fault or a defect. It is the real power available at your site, compared with the nameplate power measured in a laboratory at sea level and 25 °C.
In practice it is stated as a percentage loss: an engine derated by 20% delivers 80% of its rated output. The most widely used field rule is ≈3% loss for every 300 m above 1,000 m, plus an allowance for temperature. The rest of this guide explains where that figure comes from and how to apply it.
Why altitude reduces diesel power
A diesel engine works like an air pump. Combustion needs a fixed ratio of oxygen to fuel, and at altitude each intake stroke takes in less oxygen:
- At 2,000 m, air density is about 80% of the sea-level value.
- At 3,000 m, about 75%.
- At 4,000 m, about two-thirds (67%).
With less oxygen, less fuel can burn cleanly in each cycle, so peak power falls. Injecting the same amount of fuel anyway does not recover the power. It produces black smoke, carbon deposits and higher exhaust temperatures, and over time it damages the injector and the piston. A naturally aspirated engine cannot produce sea-level power at altitude, so the only sound approach is to plan for the loss.
How much power do you lose? The working rule
For small naturally aspirated diesel engines, the widely used engineering rule of thumb is:
Subtract about 3% of rated power for every 300 m above 1,000 m altitude, plus about 2% for every 5 °C above 40 °C ambient temperature (for hot sites).
Worked example for a site at 3,500 m:
- Altitude above the 1,000 m threshold: 2,500 m → 2,500 ÷ 300 ≈ 8.3 steps.
- Power loss: 8.3 × 3% ≈ 25%.
- A generator set rated at 10 kVA under reference conditions delivers about 7.5 kVA at that site.
These are planning figures. Manufacturers publish exact derating curves for each model, and the correction matters at the quotation stage, not after commissioning. Cold mountain air partly offsets the altitude loss, because cold air is denser, but never rely on the weather to carry your load.
Size from the site, not from the catalogue
The mistake behind most high-altitude failures is choosing the generator by its nameplate and checking altitude afterwards. Work in the opposite order:
- Add up your real load at the site: the running load plus the starting surge of the largest motor. Our generator sizing guide shows each step.
- Derate the candidate generator set for the site’s altitude and temperature.
- Compare the derated output with your load, and keep a 10–20% reserve on top.
In practice, a site at 3,000–4,000 m usually needs a generator one to two frame sizes larger than the same load would need at sea level. The extra purchase cost is small compared with a unit that trips every time the pump starts.
Derating applies to both the prime and the standby rating. A generator set that will run continuously at altitude should be sized by its prime rating, not its standby rating. Our guide on generator power ratings explains the difference.
What altitude does to the rest of the machine
Starting. Cold mornings and thin air make compression ignition harder. Engines with a high compression ratio start noticeably better at altitude. Electric start with a healthy battery matters more than at sea level, and cold-start aids such as glow plugs or an excess-fuel device are worth having.
Cooling. Thin air also carries away less heat. Air-cooled engines depend entirely on airflow over the fins, so at altitude keep the fins clean and never enclose the engine without designed ventilation. Water-cooled engines have an advantage for continuous heavy work at altitude; the trade-offs are in our air-cooled vs water-cooled comparison.
Fuel system. Some high-altitude sites also have long, cold fuel supply chains. Winter-grade diesel and clean, sealed storage prevent the waxing and water problems that are often blamed on the engine.
Water pumps. For diesel pump sets, altitude reduces the engine’s power, so flow and head fall with it. Lower atmospheric pressure also reduces the maximum suction lift, by about 3 m at 3,000 m. Size pump sets with the same derated-engine method, and place the pump closer to the water.
What to tell your supplier
When you request a quotation for a high-altitude site, give three figures: site altitude, ambient temperature range and the real load profile (running kW plus the largest motor start). A competent supplier will quote the derated output for your conditions, not the catalogue nameplate. That one exchange prevents the most common and most expensive sizing mistake in mountain operations.
HTX supplies diesel engines, generator sets and pump sets to distributors and project buyers in highland and remote regions. Every unit’s full specification is published on its product page, so you can do the derating calculation before you commit.
Planning power for a high-altitude site? Send us the altitude, the temperature range and the load list. Our engineers will size the unit for your real conditions, not for the catalogue figure. Request a quote →