What is BTU: Everything You Need to Know

What is BTU

You have to consider several factors while purchasing appliances like air conditioners, radiators, and similar. The energy or fuel requirements for these appliances are considerably important to get the expected output. However, you might find yourself in a fix when you do not understand many of these details written in acronyms. An acronym that you would come across in your purchase of heating or cooling appliances is BTU. Now, if you are wondering what BTU is, then you are at the right.

This extensive guide on what BTU is and why it is important will take you through all the essential details you require. It will help you make an informed decision when it comes to purchasing a heater, an AC, a boiler, and many other devices like these.

What is BTU?

To answer the most relevant question in this guide, British Thermal Unit, or BTU, is a unit of heat energy that an appliance can use and emit. It measures the quantity of heat energy that 1 pound of water requires to become 1°F hotter.

BTU is mainly used about heating, ventilation, and air cooling (HVAC) products as a measurement of their thermal output. In the description for an HVAC product, a BTU signifies its energy consumption in an hour for heating or cooling the surroundings. So, if an AC cools the place, then its BTU rating measures its capacity to remove heat energy. On the contrary, for a radiator, the BTU rating depicts its capacity to generate heat energy. 

Now, how does that affect your decision to purchase such an appliance? With the BTU rating, you get to know the operational capacity or thermal output range of a product. Simply put, it measures the appliance’s efficiency. In a way, it is similar to a car’s efficiency measured in MPG or miles per gallon. 

Let us understand this with an example. If you are looking to buy a boiler, for instance, then you need to know its heating capacity to pick the size that matches your requirements. The BTU rating allows you to gauge the heating capacity for different sizes of boilers. The same is true for products like radiators, air conditioners, and several other products that use and emit heat energy.

The History of BTU

“What is BTU” cannot be answered without knowing its origin. The British Thermal Unit is used around the world, and yet not many people know its history.

In the 1800s, British engineers were working on creating heat calculators. Before that, a general rule of thumb would be used to measure the heat needed.

In 1895, William Rankine, a Scottish engineer, mentioned it in his book “Steam Engine.” Soon enough, engineers started using the term “British Thermal Unit” in their everyday discoveries. By the 1900s, it became common usage for heat loss calculations and sizing boilers and heat emitters.

Why is the correct BTU rating important?

BTU is more commonly used in the UK as well as in the US. However, it is the standard measurement unit for products like radiators also. Besides, it is used for all the products that run on oil, gas, or propane. A standardized unit for BTU measurement is BTU per hour, applicable to several appliances like ACs, boilers, heaters, dryers, and more. 

The BTU rating is directly proportional to the heat energy consumption of any product. Simply put, this rating determines the product’s entire functionality in terms of its heat consumption or thermal output. With this logic, you may think a higher BTU is always better. However, that is not always the case, and this is why you must understand the right rating needed for every appliance.

A higher than needed BTU level can prove to be counterproductive. This is because such high levels of BTU can result in a shorter life span for the concerned appliance. 

How does this happen? To exemplify this with an AC, you should know that an AC works by cycling on and off as per the temperature in the room. Installing an AC with a higher than required BTU rating in a smaller room would mean that the AC would cycle on and off more repeatedly. This can definitely end up in malfunction, ultimately destroying the appliance.

On the contrary, an AC with a BTU rating lower than required for a large room will simply not do the job. It will keep running at maximum capacity and fail to function effectively because of its low thermal output. In the end, it will overwork its threshold and suffer a breakdown. This is one of the many examples to show why you need to find out the BTU rating and buy a product that fits your requirements.

What are the common appliances with BTU ratings?

If you are reading this guide, then it can only mean one thing. You are buying a new appliance that runs on gas, propane, or oil and came across the term BTU in your research. Well, knowing what is BTU is helpful if you are a homeowner. This is because several products in a home come with BTU ratings. A basic understanding of BTU and its usage with reference to some products will go a long way in determining your purchase.

Some of the most common household items that use BTU ratings include air conditioners, electric heaters, cooking appliances, solar water heating devices, and more. This section will take you through a detailed analysis of what is BTU and its reading for each of these products.

Air Conditioners

At its basic, an air conditioner removes hot air from an area and replaces it with cold air. This might create confusion for a few about the meaning of a BTU rating for an AC. Well, in reference to an AC or a heat pump, a BTU unit measures the amount of thermal energy removed instead of produced. 

The fundamental functioning of an AC involves heat collection using refrigerants. This heat is then circulated and ultimately dumped outside through the external unit of the air conditioner. The refrigerants evaporate and collect the heat present inside a room, thereby creating a cooling effect.

ACs of varying types and sizes have a different range of BTU ratings. Portable air conditioners of different sizes can produce anywhere between 8000 and 14000 BTUs in an hour. On the other hand, split system ACs have a BTU range of 9000-36000. At the same time, window ACs have a BTU capacity between 3000 and 25000. 

There are other air conditioners that come in the sizing capacity of tons. These ACs cover a larger area and are often used in a centralized manner. An air conditioner with a ton of capacity can burn 12,000 BTU in an hour. 

As already mentioned, you should choose an air conditioner that matches your requirements instead of simply going for a higher BTU rating. Many reasons can substantiate this claim. Firstly, a bigger AC will obviously cost more. Besides, its compressor will undergo damage sooner than average products. It will also add on to the humidity. It is recommended that you choose the size of an AC based on the square footage of your space. 

Cooking Appliances 

BTU is particularly relevant for burners like stoves and grills. It is a measure of the product’s thermal output capacity. This is an important factor to determine your purchase since you should be aware of the heating volume of your stove or grill. However, the BTU rating is not the only factor you need to keep in mind while finding the optimum product. Several other factors should complement the efficiency of your cooking appliance to make the best use of its BTU.

The choice of the ideal BTU depends on your primary cooking area. You should determine the ratio between the BTUs of a product and the size of your cooking space. The best BTUs-to-per-square-inch ratio is 75 to 100. You should remember that grills that have lower than the average BTUs-to-per-square-inch ratio will potentially slow down your cooking. This is because these will be inefficient and take more time to preheat. 


If you cannot imagine winters without a great radiator by your side, then you should definitely know what is BTU. The BTU rating of a radiator will give you an adequate estimate of how much heat it can produce. This knowledge can then help you choose an appliance that can fit the bill for your heating requirements. 

Without the adequate BTU level needed to warm up a room, a radiator will simply be a waste of your money. With a higher than needed BTU level, it can end up overheating your place and cause adverse consequences. Therefore, to avoid either of these situations, it is best that you understand the ideal BTU ratings for a radiator.

Several factors go into the calculation of the optimum level of BTUs required in a radiator for any area. You must first calculate the dimensions of the room and take a count of the total windows in it. An awareness of the surfaces that make the ceiling and floor of the room is also helpful. This is because different surfaces react differently to heat energy. 

Once you have all this information, you can go online on any BTU calculator and get the accurate rating level for a radiator in your desired area.  


Many homeowners keep a gas or oil furnace at their place. The gas furnaces function on natural gas or liquefied petrol gas. Now, furnaces come in an array of different sizes for different kinds of houses or commercial spaces. 

Depending on the size of your furnace, you can go for a typical BTU range of 80,000 to 100,000 units. However, finding a furnace with 40,000 BTUs is also easy. At the same time, you can get a range as high as 200,000 BTUs as well. On the other hand, a gas furnace is available in the range of 60,000 to 160,000 BTUs per hour. 

Like all other devices discussed in this guide, a higher BTU rating does not mean it is best suited for your house. Similarly, a low BTU rating furnace is also not ideal if you have a considerably large space. 

In the case of a furnace with lower than required BTU capacity, there is a high probability that the appliance malfunctions. This is because it will keep turning on and off to make up for its inefficiency. On the other hand, a furnace with higher BTUs will undoubtedly result in longer bills. It may not even do the job properly. 

The BTU requirements of a furnace vary as per the climate and several other factors. However, the ideal level of BTUs is 30-60 per hour for every square foot in an area. If you are unable to gauge the correct measurement, then it is best that you seek the advice of an HVAC professional to determine the BTU requirement for a furnace in your place. 

What are some other common uses of BTU?

BTU is commonly used as a measurement of power. This is evident when we see that most house furnaces and air conditioners are rated in BTU per hour. This unit exists to let you know how powerful your appliance will be. There are plenty of other uses for BTU beyond HVAC appliances. Let’s take a look at some of them.


Boilers are used to heat fluids and produce steam. BTU helps with boilers designed to generate warm heat or power. The water travels through the pipeline of the water heater tank. So, knowing its BTU range will determine how well equipped the boiler is for heating your home or water.

If you are looking to buy a boiler for a standard-sized home, its BTU range should be 45,000 to 300,000 BTU/h.

Fuel Energy Comparisons

BTU is also used as a unit of energy for comparing different fuels. Generally, fuels are calculated by their physical quantity. BTU is more useful when you need to calculate the energy source of the fuel. You can then compare the energy properties of different fuels like oil, natural gas, or even coal. 

That is why modern engineering publications use this unit for their research papers.

Solar Power

Technology has introduced us to solar-powered air conditioners and heaters. Where you use solar panels, you use the concept of British Thermal Units too.

By knowing how many watts they produce, you can identify the BTU usage that the solar panels can give you. You can then estimate the savings made on your solar-powered HVAC systems.

Sizing of LP Gas Appliances

Propane companies also use BTUs to determine the sizing of gas supply. The LPG appliance load is expressed in BTUs. This shows how much the system will be demanding if all gas appliances are working at full capacity.

The BTU load is how you ensure your gas appliances work efficiently. Gas supply companies use this load to select a propane tank of the right size. It is also used for selecting the size of the gas pipes and regulators.

If you have a large home with several heaters and other gas appliances, then you will require higher BTUs than one room with a gas fire pit. The gas supply company will conclude that your home requires a powerful propane appliance. So using BTU as a measurement is more beneficial for them instead of a gallon.

British Thermal Unit (BTU) And Energy Efficient Ratio (EER)

If you have not heard of EER, you will wonder what BTU and EER relation is? Energy consumption cannot be calculated just by the air conditioner’s BTU rating. For this, you will also need to know the energy-efficient rating.

The EER is calculated by dividing the BTU rating by wattage. For instance, an average 12,000 BTU air conditioner uses 1,200 watts. So it will have an EER rating of 12,000/1,200 = 10. As the name suggests, EER will indicate how efficiently your HVAC unit uses energy. So, the higher the EER, the more efficient your HVAC unit will be.

How to Figure Out Your BTU Requirement?

Now that you know what BTU is, you can understand better what your BTU requirement will be. Room specifications determine what the right kind of air conditioner or heater will be best for you. The room sizing tells you how much air will need cooling or heating.

Before buying a new air conditioner or heater, you measure the dimensions of your room. The length and width will help in calculating the exact square footage of your room. Once you are done measuring, use a BTU chart to determine how much power your appliance will need.

Your BTU requirement is not dependent on just the square footage. If you have a bigger room, it does not mean you will need more BTUs. Other factors also come into play here.


Your regional location plays a huge part in choosing the correct appliance. Hotter climates increase the difference between the ambient temperature and the desired temperature. So if you are looking for an air conditioner, you will need a powerful one. That is why countries in the South use thrice as many BTUs as countries in the North.

Room And Unit Location

Other than your regional location, you will also need to consider where your room is based. You will need to check how much direct sunlight comes into your room. If it is a kitchen, other appliances there can increase the heat in the room. If the room is on a higher story of the house, it will put more pressure on the unit.

The room location also determines the unit location. Your unit can get heated if it is working under direct sunlight and has insufficient BTUs. In such cases, you can add 10% to the average BTUs calculated for your room size.

It will help if you place the unit in a shaded or hooded area. This is usually the north side of the house or room. By installing it away from direct sunlight, you can increase its efficiency and extend its lifespan.

Some people make the mistake of covering the unit. That will reduce cooling or heating. The appliance needs to be surrounded with enough airflow for the proper distribution of the desired temperature.

Designing Of The House

Floorplans let you know about the open space in your house. Open space affects how ambient air travels across the house. So the more enclosed the layout is, the lesser energy it requires.

An open floorplan needs more power than a compact one. A house with several sprawled rooms on the same level will need more BTUs. However, a multi-story house with smaller rooms can be cooled or heated efficiently with lower BTUs.

Desired Temperature

You will be required to input the desired temperature you want for your room. Typically, anywhere between 70°F and 85°F is comfortable for most people. If the room has people coming in and out, the temperature increase or decrease will be by 45°F. This is after factoring in the body heat of the person.

Number of Occupants

This is a bonus point to make your calculations more accurate. The human body radiates approximately 100 watts of excess heat. The higher the number of people in the room, the more heat is radiated. Furthermore, excess body heat can increase the temperature of smaller rooms quickly. You can add 500 – 600 BTUs to your calculation if there will more than two people in the room.

Flooring and Ceiling Height

When looking at the floorplans, you will also learn what is below and above the room. Heat retention capabilities of a solid floor versus floors above the basement vary significantly.

The total area of the room also factors in how high the ceiling is in your room. When you calculate the BTU requirements, the BTU chart assumes the standard room size with an 8-foot high ceiling. Higher ceilings require more BTUs for a greater volume of air. If you have a higher ceiling, make sure to check out a BTU calculator that lets you add a custom ceiling height.


Insulation plays a crucial factor in maintaining the desired temperature. A well-insulated house can have the unit working efficiently on a lower amount of BTUs. If your house is uninsulated, a unit with insufficient BTUs will become costly. The unit will constantly be running to achieve the desired temperature. This will likely increase your electricity bill in the long run.

New homeowners have homes with better insulation technology. Homes that are newly built have to follow stricter building codes that include good insulation. It is recommended that older homeowners to upgrade their homes. The upgrade will keep their utility bills low and improve the overall value of the home. 

You can also buy products to help with insulation. These products mention their R-value, a measurement of thermal resistance. Thermal resistance is the property of an object to maintain heat flow. It determines the insulation of textile materials.

The product is well insulated if it is resistant to heat transfer. The higher the R-value of a product, the more resistant it is to heat transfer. So, when you are shopping for home insulation, you want to look for products with high R-value.

Room Colour

When it comes to BTU ratings, the color of the roof should not be underestimated. Darker surfaces are known to absorb more heat than lighter-colored roofs. A dirty dark roof compared to a clean, white surface will show a noticeable difference in BTU usage.


The number and size of windows contribute to the final BTU rating. Windows do not have the same insulation as well-insulated walls. Large windows require a lot of power. If you have several large windows, you will need to add 10% more to your original BTU number. To improve their insulation for air conditioners, consider buying shades or curtains for blocking sunlight.

Adjacent Rooms

What is on the other side of your room can change the heating or cooling process. Adjacent rooms affect how much heat is retained in your room. You should also know what the wall is made of when deciding on the output for your air conditioner or heater.

How Do You Calculate BTUs?

If you are new to this concept, calculating BTU can seem like a monstrous task. You do not have to worry about that. There are plenty of websites and product guidelines that provide a free BTU calculator online. Once you have gotten your measurements, you can add the input in the calculate for your final BTU number. You can then use this BTU rating to determine what appliance is best suited for your home.

BTU And Other Units of Energy

BTU is not the only measurement of energy. Calories are another commonly used unit of energy. The BTU is defined as 251 to 252 calories. If your appliances are not measured in BTU, you may be wondering – what is the BTU conversion rate? Here are some BTU conversions to help ease your process of calculations.

  • 1 BTU = .293071 Watt-hours
  • 1 watt = 3.412 BTU/per hour
  • 1 BTU = 1,055 Joules
  • 1 therm = 100,000 BTUs
  • 1 ton of cooling = 12,000 BTUs/per hour

BTU Related FAQs

1. Can A Ceiling Fan Affect BTU Usage?

Yes, ceiling fans do make a significant impact on your BTU usage. Most rooms always have at least one area with improper air circulation. Examples of this can be the corner of your bedroom or a tiny bathroom with no vent. These corners are known as dead spots of a room.

If you do not have a ceiling fan in these rooms, the thermostat will get confused with its readings. This will result in inaccurate management of temperature.

A ceiling fan lowers your BTU usage by evenly distributing the cool or hot air across the whole room.

2. Does a Higher BTU Rating Increase the Lifespan Of An Appliance?

The BTU rating is more for checking the electric efficiency of an appliance. It can have some influence over the lifespan of an air conditioner or heater. You get powerful cooling or heating that lasts longer.

If the BTU is too less, you will have to get it serviced by an HVAC technician a lot more often. Eventually, the efficiency of the heat or air conditioner deteriorates with time.

This is not uncommon and is simply an outcome of energy usage. For example, air conditioners lose over 50% of their efficiency after ten years. It is a result of liquid refrigerant becoming insufficient. 

3. Is Higher BTU Best For Large Rooms?

It can be tricky to wrap your head around this concept of power. A higher BTU means the appliance has more “horsepower.” The majority says a larger room will require a powerful cooling or heating. As a result, people choose appliances with higher BTU ratings.

At the same time, higher BTU can be inefficient too. Let’s take the example of an air conditioner.

You want your air conditioner to cool your room while also removing moisture from the air. If you get the largest unit, it will cool your home too quickly and shut off. This will stop the dehumidification process, the process that hinders the growth of mold and dust. It also is likely to wear out in the process of switching on and off.

BTUs play a crucial role in finding the right air conditioner. So make sure to choose an air conditioner that has just the right BTU rating.

Summing Up

Most of us go by living our lives without ever wondering about the efficiency of our appliances. This can affect their long-term usage. To make sure you get the most economical solution, you can check what BTU rating is for it.

BTU helps determine how consistent the fuel usage of your appliances is. You can check a BTU comparison chart or look at the product details online. The details include recommendations about room and equipment sizing.

BTU is an important factor, but not the only factor. Whenever buying your appliances, features have to be taken into consideration as well. So always consult with an HVAC technician. They will inspect your room and guide you in your process of BTU rating.