air conditioner unit outside on grass against a blue wall
ductless outdoor air conditioner unit sitting in grass against bright blue exterior wall

What Does BTU Stand For?

One crucial factor to consider when shopping for an air conditioner is the units’ BTU rating, but what does BTU stand for?

BTU is an acronym for British Thermal Units, a unit of measurement. A BTU is the amount of heat needed to raise or lower one pound of water by one degree Fahrenheit. BTUs measure energy transfer and can be used to measure both heat loss and heat gain. BTUs are used to rate appliances such as gas stove burners, ovens, furnaces, heat pumps, and even air conditioners. An air conditioner transfers energy in the form of heat from inside to outside the house. So although an air conditioner cools and a furnace heats, BTUs are used to rate both appliances.

What are BTUs in air conditioning?

An air conditioner’s BTU rating represents how much heat the air conditioner can remove from a home in one hour. Basically, the BTUs tell you how powerful the air conditioner is.

To understand how BTUs can be applied to air conditioning, it helps to have some understanding of how air conditioners work. Air conditioners don’t produce cold air. Instead, an air conditioner removes heat from the air inside a house and transfers it outside. The cooled air is then blown throughout the house through the ducts and vents. So, an 18,000 BTU air conditioner removes 18,000 BTUs of heat every hour from inside the house and displaces it outside.

illutration of a man sitting on a couching sweating while turning up ductless air conditioner

kW to BTU

Kilowatts (kW) and BTUs are both units of power. A kilowatt is equal to 1,000 watts. A kilowatt-hour (kWh) measures the energy needed to run an appliance for one hour. Power companies charge customers according to how much electricity they use per kilowatt-hour.

By converting BTUs to kilowatt-hours, you can determine how much electricity an air conditioner will use and how it will impact your energy bill. First, BTUs must be converted to kilowatts. There are .000293 kilowatts in one BTU. To convert BTUs to kilowatts, multiply the number of BTUs by .000293. Example:

18,000 BTU x .000293 = 5.274 kW

 

To determine how many kilowatt-hours your air conditioner will use in one day, multiply the number of kilowatts by the number of hours a day your air conditioner typically runs. Example:

5.274 kW x 5 hours = 26.37 kWh per day

 

To estimate the monthly energy cost for your air conditioner, first multiply the kilowatt-hours by 30, the typical number of days in one month. Example:

26.37 x 30 = 791.1 kWh per month

 

Then, multiply the result by your electricity rate. The average residential rate in the U.S. as of March 2022 was 14.42 cents. Example:

791.1 kWh per month x $0.14 = $110.75

 

It would cost approximately $110.75 a month to run an 18,000 BTU air conditioner five hours a day at a rate of 14 cents/kWh.

BTU vs. Tonnage

British Thermal Units Per Hour (BTUh) and tonnage (ton) represent an air conditioner's cooling capacity or how much heat your AC unit can absorb to reduce a room's temperature. One ton of cooling capacity converts to 12,000 BTUh. Residential air conditioners range from 1.5 to 5 tons or 18,000 – 60,000 BTU per hour.

Tonnage measures the amount of heat an air conditioner can remove from a house in one hour. The unit first came about before air conditioners existed, and people used ice blocks to cool buildings. One ton represented the amount of heat it took to melt one ton of ice.

British Thermal Units Per Hour also measures the amount of heat removed in one hour. It takes 286,000 BTUs to melt one ton of ice. Of course, how quickly ice melts depends on the amount of heat applied to it. Twenty-four hours became the standard amount of time. 286,000 BTU divided by 24 hours equals 11,916.6 BTU per hour. We round up to 12,000 BTUh and get the result: 12,000 BTUh = 1 ton.

BTU to Tons Chart

This chart shows standard BTU sizes for the equipment, including room air conditioners, mini-splits, and central air conditioner units.

Remember that mini-splits and central HVAC systems are available in fewer size options. At the same time, room air conditioners are limited to about 14,000 BTU for portable units and approximately 30,000 BTU for window air conditioners.

BTU Ton (Tonnage)

5,000 BTU

0.42 Ton

6,000 BTU

0.5 Ton

8,000 BTU

0.65 Ton

9,000 BTU

0.75 Ton

10,000 BTU

0.83 Ton

12,000 BTU

1 Ton

14,000 BTU

1.15 Ton

15,000 BTU

1.25 Ton

18,000 BTU

1.5 Ton

24,000 BTU

2 Ton

28,000 BTU

2.33 Ton

30,000 BTU

2.5 Ton

34,000 BTU

2.83 Ton

36,000 BTU

3 Ton

42,000 BTU

3.5 Ton

48,000 BTU

4 Ton

54,000 BTU

4.5 Ton

60,000 BTU

5 Ton

80,000 BTU

6.65 Ton

100,000 BTU

8.33 Ton

120,000 BTU

10 Ton

How many BTUs do I need?

Proper sizing is essential for efficient air conditioning. You should base your cooling needs on the square footage of your room or entire living space that you want to cool with air conditioning. You can figure the size of an area by measuring the length and width of your room in feet and then multiplying these numbers to calculate its square footage. Add up each room's square footage to determine your home's square footage.

Once you know the size of your area in square feet, you can reference the BTU chart below that breaks down the amount of cooling power needed to keep that space comfortable. The U.S Department of Energy (DOE) recommends 20 Btu per square foot of living space as a rule of thumb.

Home Square Footage BTUs Needed

100-150 sq. ft.

5,000 BTU

150-250 sq. ft.

6,000 BTU

250-300 sq. ft.

7,000 BTU

300-350 sq. ft.

8,000 BTU

350-400 sq. ft.

9,000 BTU

400-450 sq. ft.

10,000 BTU

450-500 sq. ft.

12,000 BTU

500-700 sq. ft.

14,000 BTU

700-1,000 sq. ft.

18,000 BTU

1,000-1,200 sq. ft.

21,000 BTU

1,200-1,400 sq. ft.

23,000 BTU

1,400-1,500 sq. ft.

24,000 BTU

1,500-2,000 sq. ft.

30,000 BTU

2,000-2,500 sq. ft.

34,000 BTU

Designed by the Air Conditioning Contractors of America (ACCA), the Manual J Calculation is the most accurate method of determining how many BTUs are required to heat and cool your space. You can request a Manual J calculation from an experienced HVAC professional.

Other Important Factors to Consider:

Although square footage estimates are generally accurate, they are rough estimates. There are other essential variables you should consider when calculating the BTUs of your air conditioner.

Ceiling Height:

Technically, you should consider the cubic volume of space and not just the room floor area when sizing your air conditioning unit. All BTU calculations and EPA recommendations are given based on square footage. These guidelines assume that your home has ceilings that are 8 feet high. However, having higher or lower ceilings will affect the air conditioner's power you need.

Climate Zone:

Heating and cooling climate zones are the same eight areas as the IECC climate zone map provided by the U.S. Department of Energy. Each climate zone is defined at the county level and based primarily on winter and summer temperatures. These zones are combined into five climate categories:

 

  • Hot-humid
  • Mixed-humid
  • Hot dry/mixed dry
  • Cold/very cold
  • Subarctic
  • Marine
U.S. Climate Zones

Zone 1
30-35 BTUs per square foot

Zone 2
35-40 BTUs per square foot

Zone 3
40-45 BTUs per square foot

Zone 4
45-50 BTUs per square foot

Zone 5
50-60 BTUs per square foot

Your climate zone plays a significant role in determining the best air conditioning system to meet your home needs. If you live in a hotter climate, the difference between ambient air and your desired temperature will be more significant, so you'll need your air conditioner to work harder. Based on climate zones alone, central air conditioner units in Florida may need twice as many BTUs as in Maine.

Experts recommend a wide range between 30 and 60 BTUs of heat per square foot. The table below shows how many BTUs per square foot you’ll need for your home based on your climate zone.

Climate Zone BTU per Square Foot

Zone 1 (Hot)

30-35 BTU/sq. ft.

Zone 2 (Warm)

35-40 BTU/sq. ft.

Zone 3 (Moderate)

40-45 BTU/sq. ft.

Zone 4 (Cool)

45-50 BTU/sq. ft.

Zone 5 (Cold)

50-60 BTU/sq. ft.

Sun Exposure:

Direct sunlight and the amount of shade your house receives are essential in cooling capacity. Direct sunlight will cause your air conditioner condenser to work harder due to the higher surrounding air temperature, consuming more BTUs. Add 10% to your BTU total for units in these locations.

Placing your condenser in a shadier area will result in greater efficiency, but it will also extend the life of your equipment. You can place shady trees and shrubbery around the outdoor unit, but consider that condensers also require good airflow for best efficiency. Ensure surrounding vegetation does not interfere with the condenser, blocking airflow into the unit and blocking it.

Insulation:

Insulation is essential to the cooling process. Living in a newer, well-insulated home may require fewer BTUs than an older house. On the other hand, an uninsulated home will need an air conditioner with a larger capacity to compensate for the loss of cool air through its walls. Thinner walls will cause your air conditioner to run longer to meet demand, leaving you with high energy bills.

If you are unsure of the insulation quality in your home, it's best to use generalized assumptions. A beach cottage built in the 1950s with no renovations should probably be classified as poor. A 5-year-old home inside a newly developed community deserves a higher rating.

Windows and Doors:

Windows and doors don't provide the same insulation as insulation walls. If you have several doors or large windows in your room, size your BTU range by 10% to ensure it has sufficient cooling ability to keep up. It would help if you considered installing double-glazed windows or adding light-blocking and insulating curtains to increase your home's insulation.

Home Shape:

A traditional ranch house will have more walls than a simple square house with the exact square footage, which means heat loss. A sprawling floorplan will need more capacity than a compact, two-story cottage. Modern homes with open floor plans will also require more power than those with smaller, enclosed rooms.

Does more BTUs mean better cooling?

Units with higher BTUs are more powerful, but don’t automatically choose a unit with the highest BTU rating. An air conditioner’s BTU rating should match the square footage of the house to cool it effectively.

An air conditioner with too many BTUs will run for short amounts of time, turning on and off frequently. Not only does this waste energy and increase your energy bill, but the air conditioner also may not be able to cool the house entirely and adequately remove humidity if it runs for less time than it’s supposed to. Frequently turning on and off also causes more wear and tear on the unit.

An air conditioner with too few BTUs also wastes energy because it will run for long periods of time without sufficiently cooling the house. Again, your energy bills will be higher and additional strain will be placed on the AC unit.

Conclusion

BTU (British Thermal Units) is a unit for measuring energy transfer. An air conditioner’s BTU rating represents how much heat it can remove from a house in one hour. Simply put, BTUs tell you how powerful the unit is.

A kilowatt-hour (kWh) measures the energy needed to run an appliance for one hour. Power companies charge customers according to how much electricity they use per kilowatt-hour. By converting BTUs to kilowatt-hours, you can determine how much electricity an air conditioner will use and how it will impact your energy bill.

British Thermal Units Per Hour (BTUh) and tonnage (ton) represent an air conditioner's cooling capacity or how much heat your AC unit can absorb to reduce a room's temperature. One ton of cooling capacity converts to 12,000 BTUh.

Proper sizing is essential for efficient air conditioning. You should base your cooling needs on the square footage of your room or entire living space that you want to cool with air conditioning. Other factors to consider include ceiling height, climate zone, sun exposure, insulation, and home shape.

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