The Department of Energy rates energy efficiency in special units known as SEERs also known as Seasonal Energy Efficiency Ratios. The minimum efficiency for manufactured HVAC equipment up to this point has been 10 SEER. On January 23, 2006, the law changed and now requires manufactures to stop producing anything less than a 13 SEER.

The higher the rating number, the more efficient the system will be which is not all bad for the consumer.
Complying with 13 SEER may mean spending more upfront, but the energy savings add up to smaller energy bills in the future. Once your home is 13 SEER efficient or higher, it's extremely likely that a savings will be  seen on your monthly utility bills. (Actual savings will depend on the age, efficiency, and condition of the unit the 13 SEER system replaces and rates of electric company.)

The government is encouraging everyone to make their homes more energy efficient. Through the Energy Policy Act of 2005 , you may obtain a special, limited time Federal Income Tax credit of up to $500 if you upgrade to qualifying high-efficiency cooling and heating equipment. (Additional information is available from the US Department of Energy and Energy Star.gov.)
We recommend you seek advise from your tax consultant to verify how this credit may apply to you.