Combined Cycle Efficiency Calculator

Work out combined cycle efficiency instantly with clear inputs, formula shown and shareable results.

About Combined Cycle Efficiency Calculator

A combined cycle recovers gas turbine exhaust heat in a steam cycle, so total output is the sum of both turbines against a single fuel input. Modern H and J class plants reach 62-64% gross. The steam turbine typically contributes a third of total output at no additional fuel cost.

Formulas

Combined cycle efficiency

Gross efficiency = (GT output + ST output) / fuel input; net = gross x (1 - auxiliary load)

Heat rate

Net heat rate (kJ/kWh) = 360,000 / net efficiency %

Worked example

With the default values below, Combined Cycle Efficiency Calculator returns Net Combined Cycle Efficiency of 53.724%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Gas Turbine Outputminimum 0 MW: 180 MW
Steam Turbine Outputminimum 0 MW: 90 MW
Fuel Energy Inputminimum 0 MW thermal: 490 MW thermal
Auxiliary Loadaccepts 0–20 %: 2.5 %

Results

Net Combined Cycle Efficiency: 53.724%
Gross Efficiency: 55.102%
Net Output: 263.250
Net Heat Rate: 6,700.9

Step by step

1Values used
2Combined cycle efficiency
3Heat rate
4Net Combined Cycle Efficiency= 53.724
5Gross Efficiency= 55.102
6Net Output= 263.250
7Net Heat Rate= 6,700.9

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is combined cycle so much better than simple cycle?

A simple cycle gas turbine throws away exhaust heat at 550-650 C. Recovering it in a steam cycle lifts efficiency from around 40% to over 60% using the same fuel.

What limits further improvement?

Turbine inlet temperature, which is set by blade materials and cooling technology, and the thermodynamic limit of the bottoming cycle. Each generation of hot-gas-path materials has added a point or two.