Efficiency & Cost
Should you buy the most efficient heat pump available - maybe not!
The Prius and Volt cars get great fuel economy, but you won't save enough on fuel over the average life of the vehicle to pay for the increase in cost. Heat pump pricing shares this characteristic.
If you have a typical 15 year old system, it was about 10 Seer new and is about 8 Seer now, due to corrosion and dirt buildup. The current Federal minimum efficiency is 13 Seer, and you can buy machines up to about 20 Seer. Power consumption will vary linearly with Seer - if 8 Seer costs $2000 a year to run, 16 Seer will cost $1000.
But the price increment for higher efficiency is not linear, the additional cost per Seer increases, as efficiency increases. This means there is "sweet spot" somewhere on the price - efficiency curve.
If 18 Seer costs $2000 more than 16 Seer, you might save another $100 a year in power, but that's a 20 year payback on your incremental investment. Not a good deal, unless you really want bragging rights and don't care about cost.
The higher efficiency machines have another drawback. The motors and controls are more expensive to replace, and the coils are bigger, so they take longer to clean. Your maintenance & repair cost will be higher.
The sweet spot will vary with dealer and pricing, but is probably about 14 Seer.
The bottom line - don't buy more efficiency than you need.