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

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)