BTU
The British thermal unit, the heat needed to raise one pound of water by one degree Fahrenheit. Equipment capacity is quoted in BTU per hour in North America, while the same figure appears in kilowatts in the United Kingdom and Europe.
Also called BTU/h, British thermal unit
What is a BTU?
A BTU is the heat needed to raise one pound of water by one degree Fahrenheit. Equipment capacity is quoted in BTU per hour, which is a rate rather than a quantity.
BTU: how it works
A BTU is a quantity of energy, so equipment is rated in BTU per hour: how much heat it can move in an hour. That distinction matters when reading a specification, because a 12,000 BTU unit is shorthand for 12,000 BTU per hour, and the same machine in a European catalogue is listed at about 3.5 kW.
Where you meet the term
- On a North American model number, where the number itself may encode capacity in thousands of BTU per hour.
- On a sizing calculation, where the heat loss or heat gain of the space is expressed in the same units.
- In a cross-market comparison, where converting to kilowatts is the only way to compare like with like.
How to check it
- Read the rated capacity from the data plate. Rated capacity is stated at specific test conditions, so the plate figure is a rating rather than what the unit delivers on any given day. Owner check
- Convert before comparing across markets. A European kilowatt rating and a North American BTU per hour rating describe the same machine, but only after conversion. Owner check
Figures you will see
| Condition | Figure | What it tells you |
|---|---|---|
| Conversion to kilowatts | 1 kW is about 3,412 BTU/h | The figure used to move between North American and European specifications |
| Conversion to tons | 12,000 BTU/h is one ton | Ton of refrigeration is the North American shorthand for equipment size |
| Common ductless capacities | 9,000, 12,000, 18,000, 24,000 BTU/h | Roughly 2.6, 3.5, 5.3, and 7.0 kW respectively |
BTU: what it is not
- BTU and BTU per hour are not the same. One is energy, the other is the rate a machine moves it at, and capacity always means the rate.
- A larger BTU rating is not automatically better. Oversized equipment on a fixed-speed system cycles more and dehumidifies less.
BTU questions
How many BTU do I need for a room?
It comes from a heat loss and heat gain calculation for that specific space, taking in insulation, glazing, orientation, air leakage, and occupancy. Rules of thumb based on floor area alone routinely oversize equipment.
How do I convert BTU to kilowatts?
Divide BTU per hour by about 3,412. A 12,000 BTU/h unit is roughly 3.5 kW, and an 18,000 BTU/h unit is roughly 5.3 kW.
Why is capacity in the model number?
Many North American manufacturers encode the nominal capacity in thousands of BTU per hour into the model number, so a model ending in 12 points at a 12,000 BTU/h unit. Confirm it on the data plate rather than assuming.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE TerminologyIndustry reference definitions for heating, ventilation, air conditioning, and refrigeration vocabulary.