I was interested so I looked up some numbers: $0.3 per kwh in California * .005Gb mp3 (3m at 128kbps) mp3 * .06 kwh/Gb for internet traffic = .00009 dollars in energy to sling 1 song to your device. Heavy margin bands there for the traffic estimate especially, but Spotify probably has a 95%+ cache hit rate so I think they would struggle to come out worse than the physical disc.