Medium Combustion Plant Directive – Calculating Thermal Input – 8th January 2024


To comply with the requirements of the Environmental Permitting Regulations (EPR) you must apply for a permit from the Environment Agency (EA) for the following:

  • New standalone MCPs between 1-50MWth
  • Existing standalone MCPs between 5-50MWth

To determine whether your plant meets the requirements for permitting it is necessary to determine the ‘rated thermal input’ of the plant.

Rated thermal input means the rate at which fuel can be burned at the maximum continuous rating of the appliance multiplied by the net calorific value of the fuel and expressed as megawatts thermal (MWth).

Most plant will have a rating plate that should give the rated thermal input, or at least the thermal output. If not, then it is possible to calculate the approximate rated thermal input using the nett efficiency.

For boiler plant this figure is 85%1

So for example if your plant has a rated thermal output of 5MW, then the approximate thermal input would be:

Rated Thermal Input = 5MW ÷ 0.85 = 5.88MWth

For generators this is dependent upon their fuel and combustion type, the chart below gives the approximate nett efficiencies to be used to calculate the thermal input:2

Fuel TypeCombustion TypePower Range (mechanical or electrical)Efficiency (ηe) (%)
Based on Mechanical PowerBased onElectrical Power
Gas Oil or other liquid fuelCompression Ignition<1MW36    33
1-5MW38    35
>5MW40    38
Natural GasStoichiometric (Rich) Burn<1MW30    28
1-5MW32    30
>5MW34    32
Lean Burn<1MW35    33
1-5MW36    34
>5MW38    36
Bio GasStoichiometric (Rich) Burn<1MW29    27
1-5MW31    29
>5MW33    31
20-50MW34    33
Lean Burn<1MW34    32
1-5MW35    33
>5MW37    35

1 Combustion Engineering Association (CEA) – Determination of the thermal input rating of a steam or hot water boiler.

2 Association of Manufacturers of Power Generating Systems (AMPS) – Determination of the thermal input power of an engine driven generator.

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