To get a measure of energy efficiency, determine your electricity use for a period and then find out the "cooling degree days" for that period. Divide KWH by degree days and compare to a time when you had the old system. The calculation will be in error if other electrical use changed significantly.

Yes, I did compare May 2010 to May 2011 & came up w/ an 8% energy savings. After replacing a 14 year old system, I had hoped for a larger savings. Is this 8% about right? And, should I be able to hear or feel a difference in the AHU's varible speeds (which I cannot) ? Or, is it possible that the AHU is operating only at 1 speed ?

May 2010 may have been a lot cooler. Bill might list average temp but that is a less than perfect measure. My gas bill was $200 for winter 2010 and $100 for winter 2011 - no changes - just a warmer winter.

Yes you should notice a difference in speed unless you never reach high stage/speed.

Does anyone even know what cooling degree days are?

I do now. Thank you very much for your help !

Your 4 ton unit is likely oversized, and is running contantly in first stage. With a single stage stat, the on time is less than the board's set (timer) upstage time. The same could be said for a 2 stage stat.

V

eh, 4 ton is on the verge of oversized for a 2000 sq foot home in Florida, with 2 stories this is likely perfectly fine. How long does your unit run while it's cooling?