But possibly less, if there's some sort of arbitrage going on. Also, the energy requirement can't fluctuate as much as the price of bitcoins does.

Note I'm not trying to be argumentative. I'm actually genuinely curious. Even just order of magnitude. Folks in the media talk about the energy cost, but never with any attempt to quantify it.

For sure. It can especially be less due to short term capital constraints. If power gets cheaper, people want to produce more but it takes time to make more ASICs. To give a real thorough answer, though, takes more numbers and possibly more expertise than I have.

It looks like the best ASICs are at around 1.5 gigahash / joule. Certainly less than 2.

So 13 million gigahash per second translates into at least 6.5 megawatts. But 2 is a pretty aggressive figure, it's probably easily above 10 megawatts.