High efficiency ratings can add up to lower utility bills. Air conditioner efficiency is measured in SEERs. The higher the SEER, the more efficient the unit, and the greater the utility-cost savings you can potentially pocket.

Start with your utility company; they can help a great deal. Comparing previous bills isn't always a good measure, as the weather is never exactly the same month to month. Instead, if you take your energy bill and divide it by the square footage of livable space in your home, don't count areas like unfinished garages or basements — you can calculate how much you are spending to heat or cool each square foot of your home. Your energy provider can tell you what the average cost per square foot is in your region for that same period of time so you can compare apples to apples.

While it may cost less initially to replace the outdoor unit on an older system, over time utility expenses will add up and may even cost you more. Replacing only the outdoor unit will lower the efficiency of the unit. Even worse, your system may fail sooner than normal and most manufacturers' warranties will be voided. You should always replace the indoor cooling coil along with the outdoor unit.

If you purchase certain energy-efficient products or a renewable energy heating and cooling system for your home, you may be eligible for a federal tax credit. ENERGY STAR distinguishes energy-efficient products.