Heat is often measured in a unit called the calorie, which is defined as the amount of heat required to raise the temperature of 1 gram of water by 1 degree celcius. The "Calorie" using capital C is different. It is the unit used to measure stored in food and it equals 1000 calories (notice this is the unit with lowercase c). Since heat is form of energy, it can also be measured using the SI unit joule or kilojoule. In the English system, heat is measured in BTU (British thermal unit), which is defined as the amount of energy needed to raise the temperature of 1 pound of water by 1 degree Fahrenheit. BTU, familiar isn't it?

BTU is the unit we use for Air Conditioners in United States. But this BTU is a little different. When talking about BTU in energy,when 3 lb of water is heated from 10°F to 20°F. 30 Btu of heat energy is added to the water, and in other words the water now contains 30 Btu of heat energy. The formula for calculating BTU is BTU = Weight in lb. x change in temperature in °F.

But when talking about AC BTU, we are not saying how much energy is stores, but how much energy/heat it can remove.Air conditioning BTU is a unit use to measure quantity of heat energy and ton of refrigeration is a measurement unit use to size air conditioner unit or refrigeration units.The terms " ton of refrigeration" goes back went they used ice as the source to remove heat. It requires 144 Btu of heat energy to melt one pound of ice at 32°F to one pound of water at 32°F.Ton of refrigeration refers to the amount of heat needed to melt 1 ton of ice in a 24 hours. 1 ton of refrigeration is equal to 2000 pound of refrigeration.