[Sawdring]

An example ~ your A/C is running. You are not running in and out and nobody is at home, yet, you don't want to come home to a hot steamy jungle that takes 6 hours to cool that evening. The A/C controller could switch over to a Relative Value of 10 degrees less than outside ambient rather than running on the Absolute Value of 70 degrees.

May I suggest a programmable thermostat? If you have any problems with homemade Minnesota designs you could find another brand, but I prefer Honeywell.

[muawiyah]

There are limits to "programable" ~ still, I have a couple of window units I've found cheaper to use in the summer than the main house unit. One of them has a remote control and I can set it to a relative value. The other one wants a precise temperature.

There's no reason at all these devices can't be built with a standard controller loaded with a "best use" program where all you'd need to do is enter time of day and zip code.

Deviations would take a bit more effort.