How to Estimate the Heater Size for a Garage

Perhaps it's Murphy's Law, but it seems that car troubles and household calamities always seem to strike when the weather turns cold. As much as you may love having an excuse to spend some time in the garage in the fall, nobody wants to be fighting serpentine belts and bleeding brakes in the dead of winter unless the garage is heated. If this is finally the winter you turn your garage into a paradise for all seasons, learn what you need to know about getting the right-size heater.

You can get a pretty accurate estimate of what size heater to buy using a few simple math formulas. Some people are good with that and don't need to take things any further, but a lot of the people who like to tinker in their garage are also the kind of people who like to understand how and why things work. For that, you need to dig a little deeper into the workings of electric and gas heaters as well as insulation.

Electric heaters work by running electricity through a piece of metal until the metal heats up. Sometimes, a fan is used to help move around this heat. This type of heating, known as infrared, warms up the heater itself and the objects around it. As the objects in your garage heat up, they help keep the space warm. Because all these random objects hold the heat, your garage gets significantly warmer even when you don't have any insulation.

Gas heaters work differently. They create heat by igniting gas and using the flames to heat an exchanger. A fan blows the warm air out of the heater and into the room to warm it. Notice that while electric heaters warm the objects around them, gas heaters don't. Gas heaters just spit out hot air, and when it dissipates, it's gone. Gas heaters do, however, warm larger spaces much more effectively than electric heaters when that space is properly insulated.

The label on an electric heater will tell you what wattage it is, giving you an idea of how much space it can heat. As a general rule, you want 10 watts per square foot of garage space. A single watt of energy is the power required to produce 1 joule of energy per second. It is calculated by measuring the amount of electrical energy inside the machine and the amount of pressure needed to move it.

1. Measure the width and length of your garage. Multiply these two numbers to calculate the square footage.
   
   
2. Multiply the square footage by 10 to determine the necessary wattage.
   
   
3. Look for a heater that meets your needs. If you're having trouble finding the proper wattage, consider converting watts to BTUs (British thermal units). One watt equals about 3 1/2 BTUs. Some heaters are labeled this way rather than via wattage.
   

Unlike electric heaters, gas heaters are never identified by wattage. They measure heat in BTUs. A single BTU is the amount of energy required to increase the temperature of a pound of water by 1 degree. Because the level of insulation in your garage impacts a gas heater's functionality, you need to take it into account when calculating heater size. You also need to remember that hot air rises, so ceiling height also matters.

1. Measure the length, width, and height of your garage. Multiply these three numbers to calculate how many cubic feet of air are in your garage.
   
   
2. Check the R-value of your insulation. If you have it, use it in your calculations. If you don't, simply rate your insulation on a scale of 1 to 5, with 1 being poor insulation and 5 being good. Use this number as a substitute for R-value if necessary.
   
   
3. Decide how much warmer you want your garage to be. This temperature rise is the difference between the typical outside temperatures in your area and the temperature you want your garage to be. If, for example, you frequently experience temperatures of 30 degrees Fahrenheit where you live and want to keep your garage at 68 degrees, you want a temperature rise of 38 degrees (68 - 30).
   
   
4. Plug your numbers into the following formula to determine how many BTUs you need to adequately heat your garage: (insulation * cubic feet of garage * temperature rise) / 1.6 = # of BTU.

5. Convert your BTUs to wattage if desired by dividing the BTUs by 3.41.