When purchasing a car, smart buyers usually check the miles-per-gallon rating.

When purchasing gasoline, they check the octane rating.

And, when purchasing a central air conditioning system, they check the SEER rating.

The what?

The SEER, or Seasonal Energy Efficiency Ratio, rating. Like its "mpg" counterpart in the automotive industry, the SEER gives an indication of the performance efficiency of the system. The higher the SEER, the more efficient the unit. And, the more efficient the unit, the lower the operating costs.

The Trane Home Comfort Institute explains that the air conditioning industry originally used an Energy Efficiency Ratio or EER to rate efficiency. This was a simple mathematical ratio of cooling output versus electrical power input.

Recently, however, the U.S. Department of Energy developed a more sophisticated test method that rates the performance of a unit over a wide range of operating conditions. The result -- the SEER -- is indicative of the unit's operation throughout an entire cooling season.

Bear in mind, however, that SEER ratings compare equipment only. Many other factors, including how you use your home and the condition of ductwork, affect energy use. That's why it's important to have a competent air conditioning dealer visit your home and evaluate your entire system.

It's also important to remember that the rating of an outdoor unit is based on a matching indoor component. As a result, if you replace an outdoor unit without replacing the indoor unit, you're not likely to get the efficiency you expect.

Experts at the Trane Home Comfort Institute say that by purchasing a system with a high SEER, you'll use less energy to cool your house, resulting in lower electric bills. In many cases, these savings are enough to partially or fully offset the cost of the new equipment within a few years.

They also recommend checking with your local utility about a possible rebate. Many utilities offer such programs based on the unit's SEER rating and cooling capacity. In most cases, the higher the SEER rating, the higher the rebate.

Finally, they point out that there's more to purchasing central air conditioning than just the SEER rating. You should also look into the reliability and durability of the system. After all, a "clunker" with good gas mileage is still a "clunker."