Check out the BOINC combined stats, which includes SETI@Home and lots of other volunteer computing projects: http://boincstats.com/en/stats/-1/project/detail/host

It says the network has about 9 million hosts, which at about 200 watts (ballpark, GPUs can consume a lot of power) would consume about 2 megawatts of electricity. Bitcoin consumes 892.54 MWh/day = 37 megawatts.