And what about infrastructure created around ASIC miners or fpga? Have you calculate it too? The system's difficulty growing too so there are constantly growing need for new Thash's ( equals new kw's).
again if we were still in the GPU arena, the network difficulty would keep on rising, because miners would always compete to gett a nice slice of the reward pie, more gpu's would be racked and stacked, like i said for 1kw a GPU unit (4gpu's) would produce maybe 4 or 5 ghash each. so even if the difficulty possibly lower over the 2 years of 2012-2014 due to maybe a 10%-20% rise instead of a 20%-40% rise. the amount of physical units would increase. like i said ASICs are a 25 fold electric saving but the amount of new GPU units would not be 25x less.. infact there would be more.
as you can see here
https://blockchain.info/charts/hash-rate the hashrate of july 2013 - august 2013 webt from 200thash to 280thash(roughly) which shows a 40% jump a month. so lets use a conservative 15% jump a month if left on GPU mining and lets work out the kw for just 1 year (july 2013-2014)
so
july 2013 200 thash (40,000kw)
gpu unit 5ghash (200units per thash x 200 thash=40,000)august 2013 230 thash (55,200kw)
sept 264.5 thash (63,480kw)
octo 304 thash (73,002kw)
nove 349 thash (83,952kw) dece 402 thash (96,545kw) janu 462 thash (111,026kw) febr 532 thash (127,680kw) marc 611 thash (146,833kw) apri 703 thash (168,858kw) may 809 thash (194,186kw) june 930 thash (223,314kw)july 1070 thash (256,812kw)
if you havnt worked out from other posts i like to grey out less neded-to-know info
so this month GPU would show only just above 1petash hashrate but a 256mw electric bill.
now lets go back to that chart the peak of this month is
145 petahash = 145mw electric bill
so asics bring 145x more hash power for 60% of the power.. hmm well thats more efficient then i first thought up.. so its all good, bitcoins use 145megawatts (to answer OP's question) compared to a GPU prediction of 256megawatts if the mining industry stuck with GPU's
now to google some stats about cities and other stuff
http://ask.metafilter.com/145849/Which-uses-more-power-Disney-or-Las-VegasAccording to this 2007 Forbes article, Las Vegas uses 5,600MW on a summer day
so the entire world of bitcoin uses only 2.6% of what las vegas does
i hate maths.. but there you have it