BTU: What British Thermal Units Mean For Your AC

BTU is an acronym that stands for British Thermal Unit, a measurement of how much energy an air conditioner uses to remove heat from indoor air. BTU typically shows how much heat an air conditioner can remove within one hour.

BTU can help you figure out how powerful of an AC unit you need based on the size of your home. When shopping for a new air conditioner, you’ll want to look for a unit with a BTU rating that matches the square footage of your home. If your air conditioner has too many BTUs for your home size, your energy costs could increase. However, an AC unit with too few BTUs may not properly cool your home.

Understanding BTU can be the key to choosing the right air conditioning system to keep you comfortable while making sure your energy bills stay affordable.

Generally, BTU is used to measure the amount of heat that’s necessary to raise or lower the temperature of one pound of water by one degree Fahrenheit. This measurement is used to quantify energy usage for many household appliances, including air conditioners, furnaces, heat pumps and cooking equipment.

When it comes to air conditioners, BTU relates to a unit’s performance. Air conditioners use a system of coils and fans to take heat from indoor air and move it outside, replacing that hot air with cooler air. BTU tells you how much energy it will take for an AC unit to do that work.

You can expect to see BTU written as a number in the thousands. For instance, an air conditioner could have 10,000 BTU, meaning it absorbs and moves 10,000 units of heat in one hour.

To convert BTU into kilowatts, which measures how much energy an appliance needs to operate, you would need to multiply your BTUs by the amount of kilowatt-hours in a single BTU. A kilowatt-hour tells you how much energy an appliance, such as an air conditioner, needs to operate for one hour. There are 0.000293 kilowatt-hours in one BTU.

Here’s what the BTU to kilowatts calculation would look like for a 10,000 BTU air conditioner:

10,000 BTU x 0.000293 = 2.93 kW 

One kilowatt equals 1,000 watts. If you’re looking to convert BTU to watts, you would multiply the number of kilowatts by 1,000. Using the example above, 10,000 BTU would equal 2,930 watts.

You can take the calculation a few steps further to estimate your energy costs. First, determine how many kilowatts your air conditioner uses each day. For the 10,000 BTU air conditioner we used in the earlier example, this is what the math would look like if the unit runs for 5 hours each day:

2.93 kW x 5 hours = 14.65 kWh (kilowatt hours) per day

14.65kWh x 30 days in one month = 439.5 kWh per month

Next, you can multiply kilowatt-hours by the local electricity rate. The average price for electricity in the U.S. was 0.14 cents per kilowatt-hour in November 2021, according to the U.S. Energy Information Administration.

439.5 kWh x 0.14 cents = $61.53 in monthly energy costs

Converting BTU to kilowatts allows you to understand how much energy an air conditioner will use in your home and how much that will cost you each month. These calculations can be helpful when determining the cost of an air conditioner beyond the sticker price of the unit.

It’s important to leverage BTUs for the size of the space you want to keep cool. Typically, an air conditioner needs 20 BTUs for each square foot of space that it’s working to cool. This can apply to an entire house or a single room. Multiply the square footage of your space by 20 to figure out how many BTUs your air conditioner would need to cool the area.

If you get an air conditioner with too many BTUs compared to your home size, you may see an increase in your energy costs. Since it has more power than what’s needed for the space, the AC unit may not run efficiently and could end up wasting energy. It would likely run for short periods of time, which could prevent the air from being properly cooled. This could create a hot, humid feeling in your house.

If you get an air conditioner with too few BTUs, the unit likely won’t be able to completely cool your space. The air conditioner may continuously run to try to achieve a certain temperature that it can’t reach. This could also raise your energy bills while creating hot and cool spots throughout your home.

As we mentioned earlier, an air conditioner generally needs 20 BTUs for each square footage of space. For example, a home that is 1,600 square feet would need a 32,000 BTU air conditioner. 

Using that rule of thumb, the following table can give you an idea of how many BTUs may be needed to cool a space based on its size. For small spaces, 5,000 BTU is as low as it gets for stationary AC units.

Keep in mind that this is an estimate. An HVAC technician would perform a more detailed Manual J calculation to determine how many BTUs your space requires. A Manual J calculation would take details like ceiling height, sunlight and insulation into account in addition to square footage.

BTU corresponds with an air conditioner’s tonnage, which describes its cooling capacity rather than its weight. Tonnage is a different way to measure how much heat an AC unit can absorb to keep a room cool.

One ton of cooling capacity equals about 12,000 BTUs. Residential air conditioners typically have a capacity that ranges from two to five tons, or 24,000 to 60,000 BTUs. You’ll also see tonnage used to describe the cooling capacity of heat pumps.

Once you know how many BTUs your space requires, you’ll be able to convert those BTUs to tons, making it easier to find an air conditioner in the size you need.

Understanding BTU is one of the first steps in recognizing the efficiency and power of an AC unit, and whether or not it’s the right fit for your home. Taking the time to check the BTU of a unit can help you make sure it will properly cool your space, keep you comfortable, and potentially save you money down the road.