
Why Are BTUs Used? For hundreds of years, BTUs have been used to measure energy and are a common unit in the heating and cooling industry. It also allows you to compare various types of fuel. For example, you can compare energy sources by converting the energy in electricity, propane, natural gas, or wood into BTUs.
Surprisingly, BTU is a very small unit used to measure the output of household appliances. In 2018, the United States consumed approximately 101.3 quadrillion BTUs of energy. That's 1,000,000,000,000,000.

What Is Efficiency, and How Does It Affect BTUs?
Efficiency is determined by the ratio of BTU input to BTU output. In an ideal world, your fireplace would convert 10,000 BTUs of fuel into 10,000 BTUs of useful heat. That, as you might expect, is not how it works. This is where effectiveness comes into play. Manufacturers rate the amount of fuel energy that is converted into output by listing a percentage.
A heater with a BTU input of 80,000 and a 65% efficiency rating, for example, will produce 52,000 BTUs. In terms of fuel or electricity, this means that the heater consumes 80,000 BTU but only produces 52,000 BTU of heat to warm your home.

Conclusion
Hopefully, you're now up to speed on the most popular unit among fireplace manufacturers. The fact that BTU is a standard unit across many different types of fuels and appliances makes comparing the various outputs much easier. Keep an eye out for the difference between the BTU input and the BTU output that has been adjusted for the efficiency of the appliance.