BTU is the measurement that most air conditioners have their capacity rated in. British Thermal Units, or BTU refers to the amount of heat required to raise the temperature of one pound of water 1 degree Fahrenheit (0.56 degrees Celsius). For comparion, 1 BTU equals 1,055 joules (the amount of energy used).

In laymen's terms, BTUs measure the amount of heat an air conditioner can remove from the air over a given period of time. The higher the BTU number, the greater the cooling capacity.

EER refers to a unit's energy efficiency rating. This number is derived by taking the air conditioning unit's BTU rating and dividing it by its wattage. For example, if a 12,000-BTU air conditioner consumes 1,200 watts, its EER rating is 10 (12,000BTU/1,200watts). Higher EER's are always preferable, however, the higher the EER, the higher the price of the air conditioner.

In order to understand whether a higher priced air conditioning unit (with a higher EER) will be worth the expense, there are several things to consider. The final decision can be based on a fairly simple math formula.

If you had a choice between two10,000 BTU air conditioning units; one with an EER of 8.3 that consumed 1,200 watts and another with an EER of 10 that consumed 1,000 watts, how would you know if the $100 price difference was justified?

In order to figure out whether or not to spend the extra money, consider the following:

• How many hours per year will you be operating this particular unit?
• How much does each kilowatt-hour (kWh) cost in your area?

If you live in a cooler climate and only plan to use the air conditioning unit in the summer for four months out of the year (six hours a day), your equation might be different than someone who lives in a milder or muggier climate. For the purposes of our example, we will assume that each kWh in your area costs $0.10. If the difference in the consumption of energy between the two units is 200 watts, every five hours the less expensive AC unit of the two will consume one additional kWh than the more expensive unit. This translates into an extra $0.10. Your formula will then look like this:

• 4 mo x 30 days/mo x 6 hr/day = 720 hours
• (720 hours x 200 watts)/(1000 watts/kW) x $0.10/kWh = $14.40

This would mean it would take approximately seven years to recoup your costs if you purchase the unit that cost $100 more.