The Calculation, Explained
Why a gas turbine shrinks on a hot day
A gas turbine is fundamentally a constant-volume-flow machine: its compressor swallows roughly the same cubic metres of air per second in January and in June. What changes is the mass in those cubic metres. Hot air is thinner; high-altitude air is thinner still. Less air mass means less fuel can be burned at the firing-temperature limit, and output falls almost in proportion. Efficiency falls too — the compressor works harder per kilogram of air while the turbine's share shrinks. This is not degradation and not a fault; it is physics, and it is why ratings are quoted at the ISO reference conditions of ISO 3977: 15 °C, 101.325 kPa, 60% relative humidity.
The model
Typical sensitivities: aeroderivative and industrial gas turbines lose about 0.6–0.9% of output per °C above 15 °C, and heat rate worsens about 0.15–0.25%/°C. Inside a combined cycle plant the steam bottoming cycle buffers the blow, so plant-level output sensitivity is nearer 0.35–0.45%/°C — use the machine's own OEM correction curves whenever they exist; they govern contractually.
Assumptions and applicability
- Linear coefficients: valid for estimating across roughly a ±20 °C band. Real correction curves bend — compressor characteristics, IGV schedules and control limits (e.g. exhaust-temperature control taking over at low ambient) are not linear.
- Below 15 °C this estimator holds output at the ISO rating rather than crediting the cold-day gain, which in practice is often capped by control limits, generator rating or the grid code. Treat cold-day capability as a separate study.
- Humidity is not modelled: its effect on output is small beside temperature and pressure (typically under 1% across the practical range for output, though it matters for NOₓ and for evaporative cooling credits).
- The altitude factor assumes output scales with inlet pressure (air density). Evaporative coolers, fogging and inlet chilling recover much of the temperature derate — evaluating them starts from the un-cooled number computed here.
Worked example (the defaults above)
A 48.0 MW machine (ISO heat rate 8,800 kJ/kWh) at 41 °C and 40 m altitude: temperature factor = 1 − 0.75 × 26/100 = 0.805; site pressure = 100.8 kPa, factor 0.995. Site output = 48.0 × 0.805 × 0.995 = 38.5 MW — a derate of 9.5 MW (−20%) — and heat rate worsens to 9,212 kJ/kWh. Anyone declaring 48 MW of dependable capacity from this machine at a Karachi summer peak is declaring weather, not capacity.
References
ISO 3977 — Gas turbines: procurement (reference conditions); ISO 2314 — acceptance tests; the standard-atmosphere relation per ISO 2533. For contract-grade numbers use the OEM correction curves with the Performance Correction Calculator.
Related reading: What a Performance Test Really Proves.
Frequently Asked Questions
How much output does a gas turbine lose per degree of temperature?
Typically 0.6 to 0.9 percent of output per degree Celsius above the 15 degree ISO reference for the gas turbine itself. Inside a combined cycle plant the steam cycle buffers the loss, so plant-level sensitivity is nearer 0.35 to 0.45 percent per degree. The machine's OEM correction curves are the authoritative values.
Why does altitude reduce gas turbine output?
A gas turbine swallows a roughly constant volume of air, so output follows air mass. At altitude the air is less dense, less mass is compressed and less fuel can be burned at the firing temperature limit. As a first approximation output scales with ambient pressure, which falls about 1.2% per 100 metres near sea level.
What are the ISO reference conditions for a gas turbine rating?
15 degrees Celsius, 101.325 kPa ambient pressure and 60% relative humidity, per ISO 3977. Ratings quoted at any other conditions must be corrected before comparison.
Related calculators: Performance Correction · Saturation Calculator