Source: http://www.coindesk.com/carbon-footprint...

Another way to look at carbon output for bitcoin mining is to go and ask a professional miner. If anyone should know how many hashes and therefore how much power it takes to compute a bitcoin, it’s an institutional miner that makes a business of turning electricity into cryptocurrency.

Dave Carlson, founder of Megabigpower, runs a massive bitcoin mining datacentre in Washington state.

10 TH/sec (10,000 GH/sec) make 1 bitcoin per day at the current difficulty, he says. His hardware uses one watt per GH/sec, meaning that it takes 10,000 watts (10kW) to run 10TH of equipment.

He runs that 10kW of equipment for a whole day to mine a bitcoin, which means that he spends 240 kW·h. That’s 24% of a megawatt hour (MW·h).

Remember that according to the IEA data, 1 MW·h of mains electricity produces 1300lb of carbon. Based on Carlson’s figures, that means that the energy he’s using would release 24% of that, or 312lbs, of carbon dioxide into the air per coin.

According to the EIA, that’s about the same as burning 15.9 gallons of gasoline, without ethanol.