Glossary · Central heating, radiators and hot water
What is radiator BTU output?
Radiator BTU output is the amount of heat a radiator gives off, stated in British Thermal Units per hour. One watt equals 3.412 Btu per hour, so 1 kW is 3,412 Btu/hr. The figure only means something at a stated water temperature, called the delta T.
Key points
- Stelrad says 1 watt is equivalent to 3.412 Btu/hr, so a 2 kW radiator is rated at about 6,824 Btu/hr.
- Delta T 50 is the UK standard output figure for domestic gas boiler systems, and lower-temperature systems use delta T 30 or 40.
- Size a radiator by the heat the room needs, not by the wall space. Oversizing wastes energy and makes temperature harder to control.
- Insulation, window size and glazing, ceiling height and how the room is used all change how much output a room needs.
- Trapped air in a radiator can make it less effective at heating the room.
How radiator BTU output works
A radiator’s output tells you how much heat it gives off. Stelrad measures it in Btu per hour, which it calls the standard measurement for the output of any heat-generating device.[1]Stelrad, Radiator FAQs Heat output can also be stated in watts. Stelrad says 1 watt is equivalent to 3.412 Btu/hr.[1]Stelrad, Radiator FAQs
To convert, multiply watts by 3.412 or divide Btu/hr by 3.412. Worked from that factor:
| Watts | Btu per hour |
|---|---|
| 1,000 W (1 kW) | 3,412 |
| 1,500 W | 5,118 |
| 2,000 W (2 kW) | 6,824 |
| 2,500 W | 8,530 |
| 3,000 W (3 kW) | 10,236 |
Stelrad treats a radiator as high output when it is usually 8,000 Btu/hr or more.[1]Stelrad, Radiator FAQs Divided by 3.412, that is about 2,345 W.
Delta T: why the same radiator has different outputs
A radiator’s output depends on how hot the water inside it is. Delta T is the difference between the average water temperature in the radiator and the room temperature. Stelrad’s example is a 20°C room with water averaging 70°C, which is delta T 50.[3]Stelrad, Delta T Explained, The Importance of Delta T & Radiators
Stelrad says delta T 50 is the UK standard for domestic gas boilers, and it also quotes delta T 30 and 40 for lower water temperature systems such as renewables.[1]Stelrad, Radiator FAQs Check which delta T an output figure uses before comparing radiators. If a system needs delta T 50 and you choose radiators rated at delta T 60, the real output will be too low.[3]Stelrad, Delta T Explained, The Importance of Delta T & Radiators Our page on flow temperature covers how hot the water is when it leaves the boiler.
What changes the output a room needs
Stelrad says to size a radiator by the heat the room needs, not by the wall space. A BTU heat loss calculator takes the room’s dimensions, insulation, window size and glazing, and how the room is used.[2]Stelrad, Radiator Sizing 101 - Calculating the Right Size Radiator A room that loses more heat, such as one with large windows, needs a higher-output radiator, for example a double or triple panel model.[2]Stelrad, Radiator Sizing 101 - Calculating the Right Size Radiator
The type of radiator matters too. Column radiators can have a different output from flat front panel radiators of the same size.[2]Stelrad, Radiator Sizing 101 - Calculating the Right Size Radiator Stelrad also warns against oversizing, because an oversized radiator can make temperature control difficult and waste energy.[2]Stelrad, Radiator Sizing 101 - Calculating the Right Size Radiator
Radiators for a heat pump need extra thought. Energy Saving Trust says heat pump running costs depend partly on whether your radiators are appropriately sized, and that installation cost depends on whether you need to upgrade them.[4]Energy Saving Trust, Air source heat pumps See air to water heat pump for how that system works. Stelrad advises talking to a competent installer about what temperatures your heat source runs at.[1]Stelrad, Radiator FAQs
Example
Gurpreet in Leeds has a living room with a large bay window. A radiator in the shop is labelled 6,824 Btu/hr at delta T 50. That is 2,000 W, or 2 kW, using Stelrad’s conversion.[1]Stelrad, Radiator FAQs The label means 2 kW only when the water averages 50°C above the room. On a low temperature system the same radiator gives less. An installer’s heat loss calculation, using the window size and insulation, decides whether it is enough.
What people get wrong
A bigger BTU figure does not mean a better radiator. The right figure is the one that matches the room’s heat loss at the water temperature your system really runs at.
Stelrad says bigger radiators do not always heat better, and that oversizing wastes energy.[2]Stelrad, Radiator Sizing 101 - Calculating the Right Size Radiator A second cause of poor output is air in the radiator. Energy Saving Trust says trapped air may make radiators less effective, and tells you to bleed them with the radiator cool.[5]Energy Saving Trust, Eight quick DIY jobs to save energy A radiator that stays cold at the bottom is a separate fault, covered in radiator cold at the bottom.
The bottom line
Radiator BTU output is heat per hour, and 1 W equals 3.412 Btu/hr. The number is only comparable when it is quoted at the same delta T. The room’s heat loss decides the size you need, so measure the room, count the windows and ask an installer for a calculation rather than buying the biggest radiator that fits. Our central heating team can survey the whole system.
Common questions
How do I convert BTU to watts for a radiator?
Divide the Btu per hour figure by 3.412 to get watts. Stelrad says 1 watt is equivalent to 3.412 Btu/hr. For example, 6,824 Btu/hr is 2,000 W, or 2 kW.
What is delta T on a radiator?
Delta T is the gap between the average water temperature in the radiator and the room temperature. Stelrad gives 70°C water in a 20°C room as delta T 50, the UK standard for domestic gas boilers.
Is a higher BTU radiator always better?
No. Stelrad warns that an oversized radiator can make temperature control difficult and waste energy. Choose the output the room needs, worked out with a BTU heat loss calculator.
Do heat pumps need different radiators?
Sometimes. Radiators are also quoted at lower delta T figures for lower water temperature systems. Energy Saving Trust says whether you need to upgrade radiators affects heat pump installation cost, and sizing affects running cost.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Stelrad. Radiator FAQs. Checked 11 November 2026. ↩
- Stelrad. Radiator Sizing 101 - Calculating the Right Size Radiator. Checked 11 November 2026. ↩
- Stelrad. Delta T Explained, The Importance of Delta T & Radiators. Checked 11 November 2026. ↩
- Energy Saving Trust. Air source heat pumps. Checked 11 November 2026. ↩
- Energy Saving Trust. Eight quick DIY jobs to save energy. Checked 11 November 2026. ↩