BITCOIN

In 2013, Mark Gimein estimated electricity use to be about 40.9 megawatts (982 megawatt-hours a day).
In 2014, Hass McCook estimated 80.7 megawatts (80,666 kW).
As of 2015, The Economist estimated that even if all miners used modern facilities, the combined electricity consumption would be 166.7 megawatts (1.46 terawatt-hours per year).
Journalist Matt O'Brien opined that it is not obvious whether bitcoin is lowering transaction costs since the costs are transformed into pollution costs, which he characterises as "environmental spillovers on everyone else, or what economists call negative externalities."
To lower the costs, bitcoin miners have set up in places like Iceland where geothermal energy is cheap and cooling Arctic air is free.
Chinese bitcoin miners are known to use hydroelectric power in Tibet to reduce electricity costs. Ownership of bitcoins implies that a user can spend bitcoins associated with a specific address. To do so, a payer must digitally sign the transaction using the corresponding private key. Without knowledge of the private key, the transaction cannot be signed and bitcoins cannot be spent.
The network verifies the signature using the public key. If the private key is lost, the bitcoin network will not recognise any other evidence of ownership; the coins are then unusable, and thus effectively lost.
For example, in 2013 one user claimed to have lost 7,500 bitcoins, worth $7.5 million at the time, when he accidentally discarded a hard drive containing his private key. Bitcoin is an open source software which was initially led by Satoshi Nakamoto.
Nakamoto stepped back in 2010 and handed the network alert key to Gavin Andresen. Andresen stated he subsequently sought to decentralise control stating: "As soon as Satoshi stepped back and threw the project onto my shoulders, one of the first things I did was try to decentralise that. So, if I get hit by a bus, it would be clear that the project would go on."
This left opportunity for controversy to develop over the future development path of bitcoin. The reference implementation of the bitcoin protocol called Bitcoin Core obtained competing versions that propose to solve various governance and block size debates; the alternatives are called Bitcoin XT, Bitcoin Classic, and Bitcoin Unlimited.