Hours of Use

• "Cooling season" refers to the time of year when you are most likely to use air conditioning -- that is, the hottest time of the year, which lasts for about three months in many parts of the United States. Otter Tail Power Company bases its estimates for air conditioner energy usage on consumers' typical usage during the cooling season. According to the company, during the cooling season consumers use room air conditioners for 40 to 240 hours per month and central air conditioners for 80 to 286 hours per month. This difference contributes to the difference in energy usage between window air conditioners and central air conditioners.
  
  

Wattage

• A 2 1/2-ton central air conditioner uses about 3,500 watts of electricity at any given point. Since an air conditioner of this size is most appropriate for a home no larger than 1,200 square feet, your air conditioner likely uses more wattage if you live in a larger home. A medium-size window unit uses a little more than one-fourth the wattage at 900 W. You need multiple window units to cool a home, but unless you have more than four, a window unit uses less wattage than central cooling.
  
  

Monthly Energy Usage

• A kilowatt-hour is the unit by which the electricity company calculates your monthly bill. You calculate kilowatt-hour usage by first converting watts to kilowatts, which you do by dividing wattage by 1,000. A central air conditioner that uses 3,500 W uses 3.5 kW and a window unit that uses 900 W uses 0.9 kW. Determine how many kilowatt-hours you use by multiplying kilowatts by hours of use. Running a 3.5-kW central air conditioner for 286 hours a month uses 1,001 kWh. Each 0.9-kW window unit you run for 240 hours a month uses 216 kWh. Four window units use 864 kWh a month, which is still less than central air conditioning.
  
  

Lowering Costs

• The key to saving energy with any appliance is to use it less. Set the thermostat on your window unit or central air conditioner to as high a temperature as you can stand during warm months so that it works less to cool your home. Sierra A. Martin of Money Allocator, a personal finance website, suggests turning the temperature up to at least 78 degrees Fahrenheit.