World's most energy-efficient desktop processor?

At 1.8GHz, the C7-D has a THD (thermal design power, aka maximum power dissipation) of 20 Watts. This makes the chip far and away the most power-hungry in Via's C7 line, which also includes the C7-M and ULV C7-M. The latter chip dissipates a max of just 7.5Watts, when clocked at 1.5GHz -- just under the generally accepted practical limit for passive cooling (fanless designs). Fanless designs save even more power, obviously, by eliminating fans. ....More

Source: linuxdevices.com