> There are widely available stats on how much energy mining takes (based on GPU energy usage, nobody's lying about this, you can run an energy meter yourself)

What does GPU energy usage have to do with Bitcoin? Isn’t it predominantly mined by ASICs? (Though GPU and even CPU is possible, but uneconomical unless someone else is paying the bill...)

And what do you think all these Ads from all sorts of websites on the web and compromised "free wifi hotspots" do with the javascript runtime in your browser?

GPU energy gives you an order of magnitude estimate using consumer hardware. These numbers are well publicized and hard to manipulate. From there you can offer a fudge factor (eg ASICs are X% more efficient) and there you go. Alternatively, you could find some posted stats on ASIC hardware, that should get you to a more robust number, but relies upon using what I consider to be more dodgy sources (someone selling ASICs or someone trying to claim higher/lower energy usage)