Last week somebody was saying they thought bitcoin is the "most efficient" way to pay, so I crunched some numbers to find out. Summary: it is not.
Currently 262 Terahash/s=262E12 hash/s
Most efficient bitfury hardware gets 705E6 hash/J, or 1.42E-9 J/hash ==>Note I'm being as conservative as possible here, in BTC's favor
So total network power consumption is at least (1.42E-9 J/hash)*(262E12 hash/s)=372040 J/s = 372 kW. Like I said, this is the most efficient scenario, with little room to improve with new mining hardware in the future. A year ago it was far, far worse.
36944 bitcoins change hands/hour, on average.
(3600 seconds * 372040 J/s)/($100/btc*36944 btc/hour)=362 J/$ processed.
Now let's compare to Visa. Visa processes avg $6.7T/year= 6.7E12 $/y = 212456 $/second. If Visa consumed as much power as bitcoin, that would be (362 J/$)*(212456 $/s)= 76.9 MW of power.
Visa has 2 datacenters in N America, consuming about 25MW each. And that includes stuff like AC, that I didn't even consider for bitcoin. So Visa actually uses far less energy/$ than bitcoin. And bitcoin is getting less efficient every day as the hash rate grows.
http://usatoday30.usatoday.com/tech/news/story/2012-03-25/visa-data-center/53774904/1Discuss.