We've got two iMacs and three Mac minis in the house. We did notice a decrease in the electric bill when we replaced (Dell) PCs with the Macs. All of the Macs except the mini server are configured to sleep when not in use. This is where the major power savings comes from. The server runs 24/7, of course, but consumes roughly 1/4 the power of the Dell computer it replaced.
These days the displays are the major power consumer. The 21.5" iMac is much more power efficient than the 27". Modern, large flat screen TVs consume as much power as the vacuum tube color TV sets of the 1950's and 1960's. So always sleep or turn off displays when not using them.