Let's do an upper bound, assuming miners are making enough to pay for their electricity (at an extremely low price), but not to pay any of their other costs (cooling, hardware, server operation, profit, etc):

(12.5 * 6) BTC/h * 2000 USD/BTC * (1/0.02) kWh/USD = 7.5 GW

Ok, that is a lot. Energy usage is on the order of a few million Americans. Sorry, I based my first reply on a debunk that doesn't sound accurate!