What Size Heater Do I Need for My Garage?

Temperatures plummeting across the nation not only means colder weather outside but frigid garages as well. This is especially true in homes that don't incorporate a central heat and air vent into these areas. Getting the right heater for your garage can be difficult, as there are different types of heaters and a seemingly complicated formula for proper garage heater sizing.

This guide walks you through what size heater you need for your garage, how to use a heating BTU calculator, and how to choose between gas and electric models.

Garage Heater Terms to Know Before You Start

Before you can calculate the size heater you need for your garage, you first need to understand a few terms surrounding these machines. Knowing this language will help you more accurately choose the heater that's best for you. A machine's power is often quantified by the amount of wattage.

Watts and Wattage

Often written in shorthand as watts, this is a unit of power that equals one joule of energy per second. In terms of calculation, a watt is measured by multiplying the amperage (the amount of electricity inside the machine) by the voltage (the amount of pressure needed to move the electricity). Most machines run off 120 volts but can have varying amperage—some larger machines use 240 volts to run.

British Thermal Units (BTU)

All in all, wattage can be a bit confusing, and what you probably want to look at instead is the number of British Thermal Units (BTU). A BTU is the amount of energy required to heat a single pound of water by a single degree—this comes out to roughly 1,055 joules. One watt equals just under three and a half BTU, which is the key conversion you'll use in any heating BTU calculator.

Knowing the number of BTUs will determine whether you have the right machine to increase the temperature of your garage. How much heat you need is determined by the temperature rise. To find it, subtract the outside temperature from the ideal internal temperature.

For example, an outside temperature of 30 degrees Fahrenheit subtracted from an ideal internal temperature of 65 degrees Fahrenheit equals 35 degrees of temperature rise.

R-Value and Insulation

From here, you also need to understand what type of insulation you have. The effectiveness of insulation is determined by the R-value. The higher the R-value, the less heat you lose over time. Fiberglass, the most common insulator in homes, has an R-value typically ranging from 2.9 to 3.8.

Gas vs. Electric Garage Heaters

There are roughly two types of garage heaters you can expect to find: gas and electric. Knowing how each works will help you decide which is right for your space.

Electric Garage Heaters

An electric heater for garage use is the most common option. Heat is generated via a metal implement—electricity passes through this point and heats up the surrounding air. The amount of heat created is equivalent to the amount of current applied to the metal squared, multiplied by the electrical resistance of that metal.

Often these machines are assisted by a fan that moves the heat around. The majority of the heat relies on infrared heating—these rays are absorbed directly into solid objects in your garage and into you, making them more effective. A garage electric heater of this kind performs better in spaces with inferior insulation.

Gas Garage Heaters

Gas heaters have a much different mechanism for heating up your garage. Propane or natural gas is fed through the heater and activated via a sparker. This burning fuel warms the heat exchanger, which activates a fan that blows hot air through the garage. This type of heater directly warms the air, making it more effective in spaces with high insulation.

There are also some gas heaters that work with an infrared tube to create infrared rays, making them function more like an electric heater.

Which Heater Is More Efficient?

Gas heaters are often much more powerful than electric ones and are usually the only way to heat a larger space, especially in really frigid weather conditions. That said, they suffer quite a bit when it comes to energy efficiency. While thermodynamics forbids any system from being 100 percent energy efficient, electric models get much closer to this than gas ones.

Upwards of 20 percent of the gas used in a gas-powered heater can be burned off, requiring ventilation if used in an enclosed space. Finally, electric heaters are usually cheaper when compared to gas models.

How to Calculate Your Garage Heater Size

There are two main ways to calculate the heater you need for your garage—either by wattage or by BTU. This is where a size calculator approach comes in handy.

Estimating Watts to Heat a Garage

Watts are easy enough to follow, as they can be quantified at 10 watts per square foot of space. For example, in an area that is 20 feet by 24 feet, you would need 4,800 watts of power (it's just the total square footage, 480, multiplied by 10). This simple method is a quick way to estimate the watts to heat a garage.

BTU Per Square Foot for Garage Heaters

Gas heaters are often rated by the number of BTU they produce, and the calculations get slightly trickier. Taking the amount of wattage you need—4,800 in our example—you then multiply it by 3.41 (roughly the number of BTUs a single watt equates to). This comes out to 16,368 BTUs needed to warm your space. Working from BTU per square foot like this is the foundation of most garage heater sizing.

Estimating BTU for Heating by Volume

For those who want a more accurate measurement and don't mind a bit more math, you'll need to calculate the volume of air inside your garage. Using our original example of a 20-by-24 garage and multiplying it by the height of the ceilings—let's say 8 feet—you get the number of cubic feet of air. So 480 square feet multiplied by 8 feet is 3,840 cubic feet.

For increased accuracy, you then factor in the R-value of the insulation. R-values on the outside of the home can range anywhere from 13 in warm climates to 27 in cold ones.

If you have really good garage insulation, you can simplify this number to 0.5; average insulation to 1; weak insulation to 1.5; and 5 if you have no insulation. Take this number and multiply it by both the cubic feet and the amount of temperature rise you want. Roughly, the formula comes out to this:

(Insulation × Cubic Feet of Garage × Temperature Rise) / 1.6 = Number of BTU

In an area of 3,840 cubic feet with a 35-degree temperature rise and average insulation (or 1), the BTU needed to heat the garage would come out to:

(1 × 3,840 × 35) / 1.6 = 84,000 BTU

To convert this back into watts, simply divide the number of BTUs by 3.41. At 84,000 BTU, this comes out to approximately 24,633 watts. The same BTU-to-watt math applies whether you're sizing a small electric unit or a full garage furnace.

Choosing the Right Heater for Your Garage

Getting the right size heater for your garage isn't necessarily difficult, as long as you know what to look out for. The difference between BTUs and watts usually mirrors the difference between gas and electric. While both types of machines work well, electric heaters are much more energy efficient.

On the other hand, gas heaters can typically cover a larger space, such as a two car garage or workshop, and may be worth pairing with professional furnace installation. When using any garage furnace size calculator or working out the BTU needed to heat a garage, you first need to know the square footage, the height of your ceilings, the level of insulation, and the desired temperature increase from the outside.

Build Your New Garage With Danley's

Danley's is Chicagoland's most trusted garage builder. They provide quality work, service, and value for their customers. Danley's has built over 100,000 garages for Chicago homeowners. Your garage will be custom made with any garage style and garage size, and you can choose from many design plans to create your dream garage.

Learn why Chicagoland homeowners choose Danley's and get a free quote online now.