SEER is simply a formula that is described by the outcome of the following equation:
Divide the system's rated BTU's by it's stated SEER rating to determine how many watts it consumes per hour, (KWH stands for kilo-watt hour).
Here's the formula (we'll use for the 3 ton unit we mentioned on the post above, 15 SEER system for this comparison):
36,000 Btu's (3 tons) (12,000 BTU's per ton), divided by 15 SEER = 2400
As already stated there's a lot more to think about including getting a replacement furnace that not only uses gas conservatively but also has an ECM motor so that whether you're running the A/C or heating it's bill for just running the blower may be 8 times cheaper then your present blower on the older furnace. Look for the ECM motor to become a legal requirement soon on all furnaces.
Gary
Autumn Air Company
(636)939-3333 or (314)838-2222