Energy & electrical
Thermal efficiency
- where
- thermal efficiency (fraction; the calculator shows %)
- useful output (kW)
- fuel input on matching basis (kW)
Explain this equation
Works out the thermal efficiency of an appliance from its useful output and fuel input. Use it to compare data plate figures or test results.
P_out is the useful heat delivered. P_in is the fuel energy supplied, on a matching basis.
Efficiency is output divided by input, the fraction of energy supplied that ends up as useful heat. The calculator shows it as a percentage.
Sense check: 21.6 kW out from 24 kW in gives 90%. A modern condensing boiler is typically around 89–94% on a gross basis.
Watch out: gross and net calorific value bases give different numbers. Figures over 100% usually mean a net basis; check before you trust or reject one.
Enter your values to see the result.
Good to know
Gross and net CV efficiencies differ. A value over 100% requires checking the energy basis; it is not automatically valid or invalid without that context.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.