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)