BTU stands for British Thermal Unit. It is the amount of heat that is needed to increase the temperature of a pound of water by a single degree Fahrenheit. It is the common heating unit used when talking about anything that heats an area. It is also used as a heating unit when it comes to air conditioners but it works in reverse than a heater. It is the amount of energy to decrease the temperature of a pound of water by a single degree Fahrenheit.   What this means for your home heating devices like a heater for your garage or shop is how much heat that this device can reliably put out. A device that is rated to put out more BTUs is one that can make an area warmer. A high rating does two things: it increases the space that a heating device can reliably make toasty and it will heat smaller spaces quicker than devices with a lower rating.   Keep in mind that high BTUs aren’t always better. While it certainly means it is more powerful, it may be too powerful for the intended amount of square feet you will be using it in. This means you will be paying for a more expensive heater than you actually need, and it is possible it will actually cost you more for your heating bill. Of course, if your rating is too low, the device will be working too hard without actually producing enough warm air for space, which will also leave you with a high heating bill even if it is not heating all that well. It is important to get a device that is properly rated for space. Slightly higher BTUs are okay. Two or three times the BTUs you need for your space is overkill and will likely cause even more problems than it solves.