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What Are BTUs?

What Are BTUs?

What Are BTUs?

One of the first things you'll run into when appliance shopping is a quirky little unit known as BTU. So, what's the deal with BTUs, and why are they so important for fireplaces and other energy-using appliances?

BTU stands for British Thermal Unit and is a measure of energy. More specifically, it refers to the amount of energy needed to raise one pound of water one degree Fahrenheit. If you want to get really fancy, you can add to the definition that the water must be at sea level and start at around 39 degrees Fahrenheit.

What Are BTUs?
Thermometer in water

Right. So, how does this help you know how much fuel you'll need or how much heat your appliance will produce?

Think of a unit we're more familiar with - the calorie. A calorie is the amount of energy needed to raise one gram of water one degree Celsius. (It's like the metric cousin of the BTU.)

We pay attention to calories because we want to keep track of how much energy is going in (fuel) and how much energy we are burning in our day to day lives. BTU helps keep track of how many energy units are in a certain amount of fuel and how many of those energy units your appliance will "burn" or use.

Often the stored energy in the fuel is converted by the fireplace or furnace into heat, so BTU can also help you determine heating capabilities. When you're dealing with air conditioners, the energy is put to work taking the heat out of the room. In that case, BTU refers to cooling capabilities.

Why Do We Use BTUs?

BTUs have been a measure of energy for hundreds of years and are a very common unit in the heating and cooling industry. It's also a way to compare different types of fuels. For example, by converting the energy in electricity, propane, natural gas, or wood into BTUs, you can compare the energy sources with each other.

Ironically, BTU is actually a very small unit for measuring the output of household appliances. In 2018, the US used about 101.3 quadrillion BTUs of energy. That's 1,000,000,000,000,000!

This is why you often see BTU numbers written with "K" to indicate "thousand." So, instead of listing the heater as having an output of 80,000 BTUs, it might simply be rated 80K BTUs.

Another way to get around listing out all those zeroes is to add in a whole new unit. Propane gas is sold based on the price of a therm. A therm equals 100,000 BTUs.

Common household appliances that measure their energy output in BTUs include heaters, furnaces, cooking units, fireplaces, etc. It's also used for air conditioners, and the higher the BTU, the more heat the air conditioner can remove from the room.

How Many BTUs Does My Home Need?

It's really important to choose an appliance that falls in the right BTU range for what your home needs. For example, a fireplace with too little BTU output will run all the time and still leave you chilly. (Nothing like watching money fly out with the electric bill and still wearing a coat inside.)

On the other hand, you don't want to overspend for an appliance that produces way more BTU than you need. And if it's a heating appliance like a gas fireplace, you risk making the room uncomfortably stuffy and hot.

Too little or too much BTU output puts a strain on the appliance and wastes money and energy. It's the Goldilocks principle: "not too hot, not too cold, just right."

What Are BTUs?
Freestanding heat appliance in a living room with a woman and her dog

So, how do you know what you need? If you can, we recommend getting professional advice to help steer you toward exactly what will work for you. In the meantime, here are some general guidelines.

Start with finding the square footage (length x width) of the area the appliance will heat or cool. In some cases, this may just be a room. Other times the unit is responsible for the whole house.

BTUs per square foot x square footage = BTU output needed

Many appliances come with charts that list how many BTUs you will need for different area sizes. These charts are helpful, but they are general estimates and don't take into account all the unique factors of your situation.

What Are BTUs?
BTU output chart

The following are several factors that could affect whether you require more or less BTUs:

Ceiling Height - Tall ceilings increase the overall volume of the room and require more energy to heat or cool the air than standard 8-foot ceilings. One tip that helps with this is to install a fan. Depending on the direction of the blades, the fan can draw hot air up and away during the summer or push the warm air back down in winter. It also helps circulate the air to other parts of the house.

Outside Temperature - It's going to take a whole lot more energy to comfortably heat a room in January in South Dakota than it will take to warm you up on a September night in Florida. Keep the climate in mind when shopping for appliances and adjust your BTU needs up or down accordingly. Some homeowners in warm climates purposefully opt for very low BTU fireplaces so that they can enjoy the beauty of the flames with little to no extra heat.

Insulation - Depending on how well insulated your home is, you may require more or less than the typical BTU recommendation. It may sound depressing, but if your house is particularly drafty or poorly insulated, you are indeed "paying to heat the outdoors" as well as your house.

Windows - Considering the number of windows goes back to the idea of insulation. A window that leaks when it rains is letting the team down when it comes to insulation. Even good windows tend not to insulate as well as a wall. Just keep in mind the number of windows you have and how well insulated they are when adjusting for how much BTU you need.

Location - If your primary purpose is supplemental heat or cooling, try to be strategic in where you place your appliance. A centrally located unit has a much easier time working to heat or cool the surrounding rooms. You may also consider directing the air from the heating or cooling system using fans and possibly ductwork.

Layout - As beautiful as sprawling ranch homes are, it's going to take more effort to keep the temperature in all the rooms comfortable. On the flip side, a compact two-story house is more efficient to heat since the warmth from the ground floor rises to the second story.

What Is Efficiency and How Does It Relate To BTUs?

Efficiency comes down to BTU input vs BTU output. In a perfect scenario, your fireplace would take 10,000 BTUs of fuel and convert it to 10,000 BTUs of useful heat. As you might have guessed, that's not how it works. This is where efficiency comes in. Manufacturers rate how much fuel energy gets turned into output by listing a percentage.

For example, a heater with a BTU input of 80,000 with a 65% efficiency rating will see a BTU output of 52,000. As far as fuel or electricity is concerned, that means the heater consumes 80,000 BTU, but you'll only get 52,000 BTU in heat that warms your home.

Now, imagine a unit that is rated at 72,000 BTU but has 85% efficiency.

(BTU input) x (efficiency number)/100 = BTU output

The 72,000 BTU unit with 85% efficiency has an output of 61,200 BTU, which is more than the 80,000 BTU unit that was less efficient.

What Are BTUs?
Natural gas fireplace BTU output rating

The loss is due to several factors, including having the heat escape before it is transferred to the room, or not using up all the energy in the fuel. Inefficiently burning the fuel can lead to more emissions, which is why efficiency is often linked to EPA emissions standards.

How To Convert BTUs?

Sometimes it's helpful to convert BTUs into other energy measurements. In case you don't have the exact conversion from BTU to watt-hours memorized, here are some of the common conversions. If you don't see the one you're looking for, it's often a google search away. Just make sure they're actually comparing BTUs and not MBTUs.

MBTU simply stands for one million BTUs, so one MBTU = 1,000,000 BTUs. It's just one more way to get around the relatively small amount that just one BTU represents.

For some other common conversions, here's a helpful list from hometips.com:

  • 1 BTU equals 252 to 253 calories
  • 1 BTU equals .293071 watt-hours
  • 1 watt is approximately 3.41214 BTU hours
  • 1,000 BTU hours equal approximately 293.071 watts
  • 1 therm equals 100,000 BTUs
  • 1 "ton" of cooling equals 12,000 BTUs per hour
  • 1 standard cubic foot of natural gas yields 1,030 BTUs


What Are BTUs?
Happy couple browsing eFireplaceStore.com for fireplaces

Hopefully, you're now all up to speed on the favorite unit of fireplace companies. The fact that BTU is a standard unit across many different types of fuels and appliances makes it that much easier to compare the various outputs.

Remember to pay attention to the difference between the BTU input and the BTU output that is adjusted for how efficient the appliance is. If the fireplace or other appliance has low efficiency, those numbers could be very different!

If you have any other questions about BTU including the BTU ratings of our products or calculating what you'll need for your home, please reach out to us! Our NFI Certified Specialists will be happy to help!

About the Author

Dr. Angela Martin

With an extensive background in advanced technical writing and academic research, Angela is our Content Editor and Project Manager. She currently supervises article production for our eFireplacestore and eCanopy brands.

Outside of work, she is actively involved in her sorority and engages in educational consulting, dissertation mentoring, and social advocacy. When she manages to get downtime, she enjoys vacationing at the beach and spending time with family and friends.

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