What Size Space Heater Do You Need?
When buying a space heater to add supplemental heat to your home, it is essential to choose the correct size heater. If a heater is too large, it wastes energy and may make a room too warm. If it's too small, it won't adequately heat the space in question.
Heater sizing is measured in either BTUs or watts depending on the type of heater. It is not related to the actual size of the unit in inches or feet. The physical size of a heater will impact its attractiveness in a room or whether it will fit into the spot you are envisioning, but heating size refers to the elements inside the heater and how much heat it produces.
Most electric heaters use watts, a standard electrical power unit. Heaters using other fuels are more likely to use BTUs or British Thermal Units to indicate the amount of power generated (technically the power rating is BTU/hour but it is nearly always listed simply as BTUs). Converting between the two units is straightforward; one BTU/hour is approximately 3.41 watts.
As a general rule, it takes approximately 10 watts to heat each square foot of room space by 10 degrees. However, this assumes ceilings of an average height, good insulation, an average amount of window space, and no vents or stairways in the space. The anticipated outside air temperature and the desired indoor temperature also play a role. For example, a 12' x 12' room with 8' high ceilings would require a 1440-watt heater to raise the room's temperature from 65° to 75° a standard 1500-watt heater would be a good choice for the room described.
Other factors affect which size heater will be adequate for a particular space. For example, each extra foot of ceiling height in a 12' x 12' room with average insulation requires about 140 extra watts of power to heat 10 degrees. If the room above had 10' ceilings instead of 8', an 1800-watt heater would more fully heat the room. A 1500-watt heater would still heat the room, but not the full ten degrees. Similarly, if you felt like the room typically needed to go from 55° to 75°, a heater with more wattage would be necessary. Poor insulation, extensive window space, or large stairwells can require even more extra power for the same results as a room without these heat sinks.
Other factors such as appliances in the room, the direction of windows, what is below the space and what is above the space can also affect the size of the heater needed to adequately warm a room. In general, large open rooms without appliances like dining rooms or lounge areas require the most heat per square foot of space. Because appliances throw off heat just by being plugged in, kitchens and bathrooms require the least. Rooms facing north require more power than rooms facing in other directions and rooms with French windows require more power than rooms with other types of windows or no windows at all. Many calculators are available online to help estimate the needs of a specific room; these calculators vary greatly in the number of factors they consider and their accuracy.
The power required to adequately heat a space has decreased over time as improvements in insulation, energy efficiency, and building materials have been phased into houses over recent years. If replacing an existing heater with a new one, the power requirements of the new heater will likely be lower than the previous heater's output (assuming the original heater was adequate for the job in its heyday). If your calculations lead to a larger heater size than previously in place, you may wish to double check the numbers or invest in a professional assessment prior to purchasing a space heater.