BTU

British Thermal Unit — an imperial energy unit (1 BTU ≈ 1,055 J) still occasionally used for radiator and boiler ratings. 1 kW ≈ 3,412 BTU/hr.

BTU (British Thermal Unit) is an imperial energy unit defined as the heat needed to raise one pound of water by one degree Fahrenheit. It survives in heating because radiator manufacturers historically rated their products in BTU/hr, and many calculators and old quotes still use it.

The conversion is straightforward:

1 kW = 3,412 BTU/hr
1 BTU/hr = 0.000293 kW

So a 1,500W (1.5kW) radiator is rated at around 5,118 BTU/hr. When sizing radiators for a heat pump retrofit in Cornwall, we always work in kW because heat loss calculations and heat pump outputs are SI-only — but the radiator manufacturer's catalogue may still quote BTU and Δt 50 (the temperature difference between water and room at which output is measured).

Watch for: a radiator rated "5,000 BTU/hr at Δt 50" only delivers around 60% of that at Δt 30 (typical heat pump operation). Always derate when planning. See our heat pump installation page.