First have an energy auditortest your home's envelope.
- Insulate and seal as well as you can as a first step toward an energy efficient home.
- You may also want to check your air ducts for leakage and seal the ducts.
- Ask an installer to calculate your cooling load after the insulation.
Once you have a good sense of the BTUs you will need, then you can hit the market for a new AC. The air conditioner'sSEER rating is key: the higher the better.
- 16 SEER is good,
- 18 SEER is great, and
- 21 SEER is the best you can get.
Whatever you choose, do not oversize the AC. It will simply short-cycle without removing the humidity. Many installers oversize the system, believing it will be better "just in case," but the result is poor performance, serious comfort issues from clammy air, and a high cost in energy and equipment.
For more information:
Also, check Steve Saunders's answer to the question "I keep hearing that I should get a home energy audit. What should I expect to pay and expect to get?"