The numbers are very back-of-the-envelope and assume a worst case (that is, widespread adoption of Bitcoin and not much improvement in Bitcoin mining activity; along with long replacement cycles for older, less efficient mining rigs), but even the best case has Bitcoin consuming a shocking amount of electricity.
Blockchain isn’t the only distributed ledger system, but it is the most power-hungry one. Other systems, like Merkle trees, can handle global-scale transaction ledgers on a couple of blade servers.
The results show that in an optimistic scenario, the increase in electricity consumption of the bitcoin network compared to now is not shocking, from around 350 MW to around 417 MW, but still on the order of one small power station. If things play out a little less favorably, however, the bitcoin network may draw over 14 Gigawatts of electricity by 2020, equivalent to the total power generation capacity of a small country, like Denmark for example.
This is by no means a comprehensive analysis and these numbers should be taken with a pinch of salt, but the conclusion is an important one: If the network of bitcoin miners keeps expanding the way