The amount of energy spent per day in an economically viable way (that is, excluding people who willingly burn more energy than what it gains them), is limited to block_subsidy*block_per_day*USD_per_BTC/USD_per_kWh.

Currently, this means around 50 * 144 * 1.7 / 0.11 kWh per day, or 111MWh/day, or 4MW power consumption. That is (currently) less than an accelerating eurostar train.

Note that assuming the block subsidy (including fees) and power price remain equal, this only depends on the bitcoin exchange rate, which may go up to 70 USD/BTC before it is economically viable to burn the equivalent of an average Boeing 747.

Also note that this is an upper limit - as long as miners want some profit, the total power consumption needs to remain below this number.