BTU stands for British Thermal Unit. It’s a measure of heat energy1 and that’s the “thermal unit” part of BTU. The “British” part comes from the use of the Imperial Gallon in the calculation. How is the Imperial Gallon used? Well, an Imperial Gallon of water weighs 10 pounds. A BTU heats one tenth of this, or one pound of water, one degree Fahrenheit. This is an older system of measurement that has been replaced in the metric system, but BTU is still used in the HVAC field.
Just as you have to add one BTU to a pound of water to raise the temperature one degree F, you could also take away one BTU and lower the temperature by one degree. It’s all about moving heat energy around. For air conditioning, we aren’t changing the temperature of water, but of air. But the idea remains the same2.
When comparing air conditioner systems, the BTU is a handy measure. There is a catch though. In HVAC, we aren’t just interested in BTU but how many BTUs can be exchanged in some set time. Although you will see equipment rated by BTU, we usually mean BTU/h (per hour)3.
Are BTU measurements accurate?
For residential applications or when comparing different types of air conditioners, they are a good yardstick. However, technically, they are not accurate. The reason is because there are many other elements that influence how hot or cold the results will be. For example, temperature changes faster when there is a great difference between hot and cold. Your hot coffee will cool faster at first and then more slowly as it approaches room temperature. Air flow is also a consideration – if cool air doesn’t circulate well, “hot spots” will resist temperature changes4.
For larger, commercial installations, the air exchange rate, mass volume and humidity all come into play. Humidity changes both how efficiently air can be cooled as well as our perception of temperature.
How much energy is a BTU?
Air conditioners start at about 5,000 BTU – for a small, window unit, and go up to 24,000 BTU or greater. To put this in perspective, we eat about 2,000 Calories a day in our food. One Calorie is 4 BTUs. So a smallish window air conditioner (8,000 BTU) is using as much energy every hour as an average adult uses in a day. Actually, it’s more, because the fan and compressor are not 100% efficient and there are losses.