Even if everyone was mining on relatively inefficient 5xxx GPUs today, 15 Thash/s would translate into ~50,000 GPUs, each drawing ~180 W of power for a total of 9 MW of power.
The Triton 9100
, a small ATM, draws 230 watts. There are approx 2.2 million ATMs worldwide
, drawing a total of 506 MW of power. In other words, 56 Bitcoin networks.
Interesting comparison, but I don't think it's meaningful. ATMs are only a tiny technical part of government-issued fiat economy. Combined electrical power of all payment processing centres is likely orders of magnitude higher, but then again this economy is orders of magnitude higher than current Bitcoin economy. 56x in this conext is irrelevant, might as well be 0.1x or 560x.
There have been threads and Wiki entries discussing overall efficiency and scaling of Bitcoin network, comparing it to Visa or similar processors. My impression is that, overall, Bitcoin can be efficient.