Well running my rig on both WCG and GPUGrid would get into the 700-800W ballpark. Not going to run that 24/24, the electric bill wont be looking good at all... granted it's definitely NOT built for energy efficiency. Idle is above 200W already..
The one i SHOULD be running 24/24 is an atom+ION 24W (at full load) HTPC, but it's totally not suited to crunching (let's say the CPU is barely more efficient than a handheld calculator...).