Generator altitude derating is the loss of output a generator suffers when it runs above sea level. Air gets thinner as elevation rises, and thinner air carries less oxygen into the engine cylinders. With less oxygen, the engine burns less fuel and produces less power.
Thinner air at elevation carries less oxygen into the cylinders, reducing combustion efficiency and engine output.
Manufacturers rate gensets at 25 °C, 1,000 mbar, and 30% humidity per ISO 3046-1 and ISO 8528. Real sites differ.
Above roughly 1,000 m, output falls by about 1% for every 100 m of additional altitude on a turbocharged diesel engine.
A 500 kVA set that delivers full power at a coastal site will not deliver 500 kVA in La Paz at 3,640 m, or in Bogotá at 2,640 m. You must size for the conditions where the generator actually runs — and the generator altitude derating calculator does that in one pass.
Less oxygen per cylinder stroke means less fuel burned and less power produced — regardless of how new or well maintained the set is.
Nameplate ratings assume 25 °C and sea-level pressure. High elevation is the most common condition that derates a set below its label.
Turbo engines compress thin air before combustion, so they hold power better: roughly 1% per 100 m above 1,000 m.
The generator altitude derating calculator applies altitude, temperature, and humidity — so you get the real power your site will actually see.
Enter your generator rating or connected load, plus your site elevation, temperature, and humidity. The calculator returns the real power you can expect at your site and the derated size you need to order.
Enter the rated output at sea level and your site conditions. The calculator will show the derated watts available at your elevation.
Enter the sea-level rating or your required site load, plus site conditions. The calculator returns derated kVA and kW, and recommends a real generator model.
The loss is a percentage of rated output, and it grows with elevation. For a modern turbocharged diesel genset, the ISO-based rule is 1% per 100 m above 1,000 m. Below 1,000 m, most manufacturers apply no altitude derating at all.
| Site Altitude | Derating (turbocharged diesel) | Output Remaining |
|---|---|---|
| Sea level to 1,000 m | 0% | 100% |
| 1,000 m to 2,000 m | 1% per 100 m above 1,000 m | 90–100% |
| 2,000 m to 3,000 m | 1% per 100 m, plus 0.5% margin | 80–90% |
| 3,000 m to 4,000 m | 1.5% per 100 m (high-altitude turbo) | 65–80% |
| Above 4,000 m | Special high-altitude engine required | Case by case |
This is the question most engineers arrive with: given a real site, what size do I buy? It's a reverse calculation. You start with the load your site needs at elevation, then work back to the sea-level rating that will actually deliver it.
Determine the required load at your site in kW, then convert to kVA at a 0.8 power factor. This gives you the true capacity your generator must supply at elevation.
Calculate your total derating by adding altitude, temperature, and humidity losses. The generator altitude derating calculator applies all three factors together in one pass.
Take your site load requirement and divide it by the remaining capacity after derating. This gives you the sea-level rating needed to deliver your required power at elevation.
Add a 10 to 25% reserve for future load growth and non-linear equipment such as VFDs, UPS systems, and LED lighting that add extra kVA demand.
Map the final result to a standard generator rating and have an engineer confirm it. Never rely on a sea-level nameplate for a high-altitude installation.
At 0.8 power factor, 800 kW = 1,000 kVA of load. Combined derating at 3,000 m and 40 °C is roughly 24%, leaving 76% of rated power available. Divide 1,000 kVA by 0.76, then add reserve. The correct specification lands in the 1,250–1,500 kVA class. A sea-level 1,000 kVA set would be overloaded on day one.
Engine technology decides how fast the loss accumulates. The difference between naturally aspirated and turbocharged engines can double your derating at high elevation.
Draws air at ambient pressure. Power drops sharply with elevation — 3 to 3.5% per 1,000 ft above 1,000 ft. At 3,000 m, an NA engine loses roughly double what a turbo unit loses.
Compresses thin air before combustion. Holds power far better — 1 to 2% per 1,000 ft. Many turbo sets run at full rating up to 3,000 m, making them the clear choice for high sites.
If you have a choice, specify a turbocharged set for any site above about 1,500 m. The premium for the turbo option is small compared with the derating you avoid.
Engine partners including Cummins, Perkins, Weichai, and Yuchai publish different derating curves for each engine family. The calculator lets you pick the policy that matches your unit.
Alternator output also derates with heat and altitude: roughly 3% per 10 °C above 40 °C and 0.4% per 100 m above 1,000 m for a typical Leroy-Somer or Stamford machine.
The effective rating of the genset is the more severe of the two limits. Whichever loses more power — the engine or the alternator — is the number you must design to.
Our engineers see the same five errors on high-altitude projects repeatedly. Avoid all of them and your set will run at nameplate conditions instead of failing at the worst moment.
Hot sites lose more than cold sites at the same elevation, and the losses add together. A 45 °C day at 2,000 m can add 4% to your altitude loss — enough to undersize your set.
At 3,000 m, a naturally aspirated engine loses roughly double what a turbo unit loses. The turbo premium is small; the derating you avoid is large.
The nameplate number is only valid at reference conditions — 25 °C and sea level. The derated number is what your site actually gets. Size on the site number, not the label.
The alternator can be the limiting component, not the engine. At high heat and altitude, the alternator may lose more than the engine — and that becomes your effective rating.
High-altitude projects grow. Non-linear loads — VFDs, UPS, LED lighting — add a need for extra kVA that a sea-level calculation misses. Always add 10–25% reserve.
Not sure your size is right? Our engineers will review your load list and site conditions and confirm the derated rating before you spend anything. Ask our engineers.
You’ve calculated your derated kW and kVA. Now let our engineers verify your site conditions and recommend the right generator for your elevation. With 25+ years of experience and a full range from 8 to 4,000 kVA, we build diesel gensets for the world’s toughest high-altitude sites — Andean mines, Tibetan telecom towers, mountain construction projects, and remote RV locations.
A: Thinner air at elevation carries less oxygen, so the engine burns less fuel and produces less power. Output falls roughly 1% per 100 m above 1,000 m for a turbocharged diesel, and 2 to 3.5% per 1,000 ft for a carbureted portable set.
A: A typical portable generator loses about 15% of its rated output at 5,000 ft, roughly 3% per 1,000 ft above sea level. An EFI engine loses less, and a jet-kit re-tune recovers most of the loss on a carbureted unit.
A: Yes. Output drops about 1% per 10 °C from 25 to 40 °C ambient, 2% per 10 °C from 40 to 50 °C, and 3% per 5 °C above 50 °C. Temperature derating adds to altitude derating rather than replacing it.
A: Altitude derating comes from low oxygen at elevation. Temperature derating comes from low air density in heat. Both limit combustion, and both reduce output. The generator altitude derating calculator applies them together as additive factors.
A: Yes, but far less than naturally aspirated engines. A turbo unit loses roughly 1 to 2% per 1,000 ft above 1,000 ft and often holds full rating to 3,000 m, versus 3 to 3.5% per 1,000 ft for a naturally aspirated engine.
A: Add the altitude, temperature, and humidity losses as percentages, then subtract the total from 100%. For example, a 500 kW set with 14% combined derating delivers about 430 kW at your site. The calculator does this for you.
A: Take your required load, convert kW to kVA at a 0.8 power factor, and divide by roughly 0.76, the power remaining after altitude and temperature derating at 3,000 m. Then add a 10 to 25% reserve. Run the calculator for your exact numbers.
A: Yes. We build high-altitude and special-engine configurations for extreme sites, and we can derate and confirm any model in our 8 to 4,000 kVA range for your exact site conditions before you order.
