Air conditioners are rated by the amount of watts of electricity required to run them. A rule of thumb is that small air conditioners use less electricity than larger air conditioners, and central air conditioning units require much more energy than window air conditioning units. Because a greater use of watts relates directly to a higher electric bill, consumers are, understandably, interested in how many watts are used by an air conditioner they consider purchasing. Of course, how many hours per day the appliance runs also makes a difference in the wattage it uses.
The smallest window air conditioner you're likely to find on the market uses about 500 watts of electricity per hour. A medium-size unit takes the usage up to about 900 watts per hour, and a large air conditioner draws about 1,400 watts per hour. When comparing these numbers and weighing the progressively higher cost to run larger units, it makes sense to measure the square footage of the area you want to cool and buy an appropriately sized unit instead of one that is too big.
A central air conditioning system is usually intended to cool an entire house, thus taking the place of several window air conditioners. Along with this increased cooling power comes a greater wattage consumption. A one-ton central unit uses about 1,900 watts of electricity per hour, while a mega-sized five-ton model uses about 9,200 watts. Depending on how many window units it takes to keep your house cool, a central air conditioning system actually may be less expensive to operate than multiple window air conditioners.
Though wattage typically is printed on an air conditioner or an attached tag, environmental conditions can cause a fluctuation in the appliance's actual electricity usage. If the unit is in the shade of a large tree, it will run cooler than if exposed to direct sunlight because it doesn't have to work as hard, thus drawing fewer watts of electricity. The quality of wall insulation in the room to be cooled also has some bearing on the air conditioner's real-world electricity consumption.