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Gas Turbine: ISO Rating and Site Rating

Gas turbine performance will vary depending on the environmental condition, i.e., ambient temperature, pressure, altitude and humidity. Changes on the ambient temperature, for example, will impact to the power developed by gas turbine. For the purpose of comparing gas turbine from each manufacturer, ISO condition is defined and the manufacturer normally will provide their performance map in ISO condition.

ISO condition refer to the following:

  • Ambient temperature: 15 0C (59 0F)
  • Altitude: 0 meter (sea level)
  • Ambient Pressure: 101.325 kPa (abs) or 29.92 in.Hg
  • Relative Humidity: 60 %

The other item that impacts to the gas turbine performance and manufacturer will include in their calculation are the fuel used (fuel quality), inlet and outlet losses (pressure drop occurs in inlet filter, plenum, exhaust stack, silencer, etc.), and power take-off include any devices that directly driven by gas turbine output shaft.


As ISO condition is not reflecting the site condition where gas turbine will be installed therefore, we will see the term “Site Rating” which is the power that gas turbine will be developed at the site installation. The correction factor will follow the chart in Fig-1 and Fig-2 below. 


Heat rate is not affected by altitude.

Correction factor for power from 1 at 0 m (sea level) to 0.8 at 1800 m above sea level, at 1000 m is about 0.89.

Inlet losses

Correction factor Heat rate is 0 for 0 mm H2O inlet losses to 1.018 at 300 mm H2O.

Correction factor Power is from 0 for 0 mm H2O inlet losses to 0.953 at 300 mm H2O.

Exhaust Loses

Correction factor Heat rate is 0 for 0 mm H2O inlet losses to 1.02 at 300 mm H2O.

Correction factor Power is from 0 for 0 mm H2O inlet losses to 0.98 at 300 mm H2O.


Correction factor for Heat rate is from 0.92 from -29 0C to 1.1 at 45 0C.

Correction factor for Power is from 1.25 from -19 0C to 0.83 at 40 0C.


Slightly changes on heat rate but no changes on power output - from 60 to 100 % humidity. Correction factor for heat rate is 1.0016 and for decrease to zero is 0.9979.

Fig-1 Altitude and Inlet Losses Correction Factor (Source: GPSA Handbook).

Fig-2 Exhaust Losses and Ambient Temperature Correction Factor (Source: GPSA Handbook). 


Sample calculation 

Let we assume we have the following ISO power for certain gas turbine model and their site condition installation:

  • Turbine ISO Power: 26,300 KW
  • Ambient Temperature: 33 0C 
  • Altitude: 100 m
  • Inlet Pressure Drop: 140 mmH2O
  • Exhaust Pressure Drop: 155 mmH2O
  • Relative Humidity: 90%


From the above chart we can expect the following power correction:

  • Ambient Temperature: 0.889 
  • Altitude: 0.980
  • Inlet Pressure Drop: 0.978
  • Exhaust Pressure Drop: 0.990
  • Relative Humidity: no correction for power 


Based on above correction factor, calculate Site Rating (Site Power) by multiplying ISO Power by each correction factor:

  • Site Power: 26,300 KW X 0.889 X 0.980 X 0.978 X 0.990
  • Site Power: 22,185 KW