https://www.theguardian.com/technology/2018/nov/05/energy-cost-of-mining-bitcoin-more-than-twice-that-of-copper-or-gold

New research reveals that cryptocurrencies require far more electricity per-dollar than it takes to mine most real metals

The amount of energy required to “mine” one dollar’s worth of bitcoin is more than twice that required to mine the same value of copper, gold or platinum, according to a new paper, suggesting that the virtual work that underpins bitcoin, ethereum and similar projects is more similar to real mining than anyone intended.