What’s a BTU? Heating Units 101
There’s no better place to examine units than the drink aisle of the grocery store: a 12-oz bottle of soda, a 2-liter jug of lemonade, a gallon of milk. Liquid ounces, liters, gallons, pints, and cups are all measure volume. We have an intuitive feel for how much liquid each one contains, acquired over a lifetime of ordering coffee, reading recipes and grocery shopping. Units of heat, on the other hand, are a little less ubiquitous.
Heat is energy, something that you feel rather than see in a gallon-sized container. Nonetheless, it can be quantified, and has been for centuries. But the unit used to quantify heat will vary widely depending on the place and application.
Many scientists use the unit Joule to measure all kinds of energy, including heat. In fact, the Joule is the official standard unit for energy in the International System of Units, or units that are used throughout the physical science fields. The Joule gets its name from an English scientist, James Prescott Joule, who studied heat and work. For people familiar with electricity, you can think of a Joule as the amount of energy needed to produce one Watt for one second. There are 4.184 Joules in one calorie–just like there are 4 quarts in a gallon.
We’re all familiar with calories, but not necessarily in the context of heat. One Calorie – capital ‘C’ – is 1,000 calories (lowercase ‘c’), and is used to describe the amount of energy stored in a given food. A heating calorie is defined as the amount of heat required to raise one gram of water by one degree Celsius. In fact, the name calorie comes from the Latin word ‘calor,’ meaning heat. As for a food Calorie, you can think of it as about the amount of energy contained in a single radish.
In the context of heating your home, you’re more likely to encounter the BTU or kWh. You can find these units on your AkWarm home energy rating describing how much heat your house needs on the coldest day of the year. They also appear in manuals for heating appliances to describe how much heat your boiler, furnace or stove can produce. A BTU, or British Thermal Unit, is roughly the amount of heat produced when you burn one match. Technically, a BTU is the amount of heat required to raise one pound of water by one degree Fahrenheit.
The kilowatt-hour, kWh, is often used in reference to electrical energy–whether you’re talking about heating your home or brewing a pot of coffee. Since energy is energy, no matter what form it takes, a kWh can also be used to quantify an amount of heat. This is especially convenient when heat is produced by an electrical appliance, such as electric baseboard.
The kWh is defined as 1,000 Watt-hours, or the energy used when a 1,000-Watt appliance is used for one hour (about the energy needed to run a coffee pot for one hour). It is a bigger “unit” of heat than a BTU, Joule or calorie, just like a gallon is larger than a quart. In fact, one KWh is equal to 3,413 BTUs, or 3.6 million Joules.
Of course, there are other units for heat energy, like the therm, quad or erg. Each has its own application, but in the end, all are different ways of quantifying an amount of energy.