What Does BTU Stand For — and What Does It Mean for Your Home?

If you've ever shopped for a portable air conditioner or a space heater, you've probably noticed — tucked among a long list of numbers and abbreviations on the spec sheet — three specific little letters everywhere you look: BTU. The average person doesn't use this acronym much in daily life, so it can be a little confusing to see it pop up everywhere you turn when you're thinking about heating and cooling your home.

So how important are BTUs, and what do you need to know about them? We've got all the answers, explained for regular people — no need to be a science whiz or an engineer to figure it all out!

So what does BTU mean? BTU stands for British Thermal Unit, though the term isn't only used in the United Kingdom. It's a measurement that's part of the common system, along with things like pounds, inches and degrees Fahrenheit. You'll hear about BTUs anywhere these forms of measurement are used, but mostly in the United States. (This is in contrast to Europe, where the metric system is the default.)

The British Thermal Unit is a way to measure energy. One BTU is the amount of energy it takes to raise the temperature of one pound of water by one degree Fahrenheit at sea level. Technically, this measurement refers to the amount of energy this process takes when water is at its greatest density, which usually happens at about 39 degrees Fahrenheit.

A BTU is the common system's version of a calorie. Calories are a measure of energy in the metric system, a measurement of the energy required to raise the temperature of one gram of water by one degree Celsius. When you count calories, you're keeping track of how much energy you put into your body (and need to burn off to maintain your current weight). When you count BTUs, you're keeping track of how much energy your appliance can process to heat or cool your room.

As mentioned, BTUs measure the amount of energy it takes to produce a certain amount of heat. When it comes to an air conditioner or heating system, the BTU rating tells you how powerful your appliance is. For example, a heater that is labeled as a 5,000 BTU heater can produce 5,000 BTUs of energy over the course of an hour. This is technically known as the BTU/hr rating, but many packages simply call this BTU.

A heater with a higher BTU rating is more powerful — that is, it has a higher heat output — than one with a low BTU rating. It can do more to raise the temperature in your room each hour, so you can either heat a room more quickly or heat a larger space.

BTUs are used to measure energy in relation to heat (remember, it originally comes from talking about heating a pound of water up), so it makes sense to label heaters this way. But what about air conditioning? In this case, you're trying to cool a room, so how do BTUs fit in?

Air conditioners actually work by removing heat from a room, rather than adding cool air. This is enabled by the compressor system, which uses a chemical refrigerant to absorb heat and then carry away from your room, where it is released into the outdoors. In this case, BTUs aren't a measurement of how much heat is added, but rather how much energy it takes to remove the heat using the compressor system. It still takes energy to do this, so the BTU is a measure of this energy rather than of direct heating.

As with a heater, high BTUs on your air conditioner means that it has a bigger capacity to get its job done. If you're looking to compare cooling output for similarly priced air conditioning units, looking at the BTU rating can give you an idea of which one is the most powerful and will give you the biggest bang for your buck.

BTUs are a very important factor when it comes to choosing the right size heat pump, space heater, or cooling system for your home. For example, if you choose an air conditioner with insufficient cooling power, it will be running constantly, driving your electricity bills through the roof and leaving you unpleasantly hot and sticky.

When air conditioner experts talking about "sizing" a cooling unit, they don't mean using a tape measure to see if it will fit in your window. Instead, they're referring to a particular unit's cooling capacity as measured in BTUs. Higher BTU ratings mean a more powerful air conditioning system, but you also don't want to overspend if you don't need to.

How many BTUs you need to cool your room depends primarily on the size of the space; that is, how much air you have to cool. You can figure this out by measuring the length and width of your room and then multiplying these numbers together to determine its square footage. Once you know your room size in square feet, you can consult a BTU chart that breaks down the amount of cooling power needed to keep that space comfortable. There's a bit of a BTU "start-up cost" when it comes to cooling, so you'll find that a small room of 150 square feet will require 5,000 BTUs, but a room three times as big doesn't require three times as many BTUs to stay cool.

There's more to sizing an air conditioner than just square footage, though. Consider these other factors to adjust your BTU total before you shop for an air conditioner:

• Ceiling Height: Technically, it's the total volume of air to cool and not just the floor area of the room that you should take into account when sizing your unit. When BTU recommendations are given based on square footage, they assume a standard, 8-foot ceilings. If you have high ceilings, however, your volume of air will be greatly increased and require a bigger BTU rating. You can avoid this by using a BTU calculator that factors in ceiling height. 
• Climate: If you live in a warmer climate, the difference in the ambient air and your desired temperature will be greater, which means you'll need your air conditioner to work harder. Central air units in the Deep South may need twice as many BTUs as those in New England. When it comes to air conditioner size, your region matters. 
• Unit Location: Direct sunlight will cause your unit to work harder, as will having it on a hotter second or third story of your house. Add 10% to your BTU total for units in these locations. Likewise, an air conditioner in a kitchen filled with appliances will be dealing with a larger heat load and will therefore need to be bigger — typically by about 4,000 extra BTUs.
• Insulation: Insulation is crucial to the cooling process. If you live in a well-insulated house — usually a newer home — you may be able to get by with lower BTUs than a house with average insulation. An uninsulated home, on the other hand, will need a more powerful air conditioner to keep up with the loss of cool air through the walls. Otherwise, the air conditioner needs to run all the time to keep up with demand, leaving you with sky-high energy bills.
• Windows: Windows generally don't have the same insulating power as insulated walls, so they detract from your overall insulation rating in your home. If you have many large windows in your room, size your BTU range up by 10% to make sure it has sufficient cooling ability to keep up. You can also consider adding light-blocking shades or insulating curtains to help boost your windows' insulation.
• Home Shape: The more compact your home's layout, the less energy it takes to cool it. A ranch house with several wings has a lot of wall area, which means more potential for your cool air to dissipate. A home with the same square footage arranged in a multi-story floor plan has a more compact footprint and won't lose its cool air as quickly. In general, the more sprawling the house, the more BTUs you'll need to keep it cool. Open floor plans also require more BTUs than enclosed rooms, which are chilled more quickly.

To choose the right room air conditioner for your space, a good rule of thumb is to take its square footage and consult a BTU chart or check the package for recommendations. If you have high ceilings, lots of windows, or poor insulation, choose an air conditioner at the top of the BTU range recommended for your room size, or bump up a size to be safe. These recommendations will work for portable air conditioners, window units, and space heaters, though you should always consult a professional when it comes to the complexities of sizing a whole-house HVAC system for heating and cooling.