Window air conditioning units help people who do not have a central cooling system stay cool when temperatures rise.

When shopping for window air conditioners, consumers will no doubt encounter the acronym BTU. BTU stands for British Thermal Unit, which is a traditional measurement of heat, defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at sea level. While BTUs may make sense when determining heat output of gas grills or the warming capacity of a heating appliance, just what do heating units have to do with air conditioners?

According to Compact Appliance, when BTUs are used in relation to air conditioning systems, the measurement expresses how many BTUs can be removed from the air per hour, essentially reflecting how well heat can be extracted from a room and cooled by the unit.

While it may seem like having the highest BTU rating would always be ideal, this isn't always the case. Cooling a room too quickly with an oversized unit may result in the appliance having to cycle on and off frequently, eventually overworking the air conditioner's compressor and potentially shortening its life span.

Similarly, using a unit that doesn't have enough BTUs will prevent the air conditioner from making a room comfortable, potentially compelling users to run the appliance in excess.

Instead, consumers need to find the right unit for their needs. Measuring the square footage of a room and then comparing it against BTU guidelines for particular air conditioner units is a great way to ensure you find the right window unit. According to the home improvement experts at Lowes, the following are the advised BTU ratings matched up with room sizes:

· 150 to 350 sq. ft.: 5,000 to 8,000 BTUs

· 350 to 550 sq. ft.: 8,000 to 12,000 BTUs

· 550 to 1,050 sq. ft.: 12,000 to 18,500 BTUs

· 1,050 to 1,600 sq. ft.: 18,500 to 25,000 BTUs.

Other factors will influence cooling capability as well. Consider whether a room gets a lot of sun or shade, and then adjust the BTU rating accordingly. Second-story rooms may have more ambient heat. Also, the number of occupants in a room may drive up the temperature, which may require purchasing a large unit.