Heating/cooling only the space needed in a home has quick paybacks and saves lots of energy.

I have central air/heat and installed a small window AC to be used with a 5kw generator during power outages (some of them have lasted 5 days). The 5000btu window cost about $135 at Lowes and the generator could easily run a couple of these, a refrigerator, freezer, few lights and television. So to try out the window AC in my bedroom (without generator) I turned the central air up to around 85 (for the house) and set the window unit for 70. Then I installed a Kill-a-watt meter to measure the energy usage. Max watts was 550 and it used only 2.2 kwh for the night (about 9 hours), and I was completely comfortable. This showed me...why not use this when the kids are not home and save electricity. My electricity bill dropped to levels I had never seen before in this home...and yet I was still comfortable as I wished. Payback of the window unit was 1-2 months from the reduced electric bill. I am now shopping for a small heater to do the same for winter.

I also installed a small window AC in my mother's home (she too has central air/heat) and she too saw huge results also in her electricity bill, while loosing no comfort. I know this seems simple...but until I ran my "test", I had never thought or read of it.
Kent