Legit Reviews recently compared the power consumption levels of an ASUS P5E3 Premium WiFi-AP@N Edition to the Gigabyte GA-X48-DS5 motherboard. In the article they discovered that the ASUS P5E3 Premium was more efficient, but declared consumers the overall winner as both boards lower power usage. Since the review was posted they have gotten negative feedback about the boards they used for testing and have a new motherboard and new test results!
"The ASUS P5E3 Premium/WiFi-AP@n motherboard uses the EPU together with AI Gear 3+ to help attain the best power efficiency and energy savings we have seen to date. Even with no software enabled the ASUS board uses 9% less power at idle and 6.5% less under load, which is awesome. Is ASUS the hands down winner though? It really depends on what you are after. For a computer system that is used often the Gigabyte DES utility is superior as it is fully dynamic and you don't sacrifice performance when you need it..."