I found an interesting sector diagram at Greentech Media:
It has the unfortunate problem of calling "Greentech" what the rest of the world calls “Cleantech” (OK, I’m biased). Still, I thought it worth bookmarking as an interesting taxonomy on how to think about all the various segments within the sector.
I don’t think that the NAICS or SIC will ever reclassify all these disparate industries into a common sector — efficiency will remain up with appliances and devices, while power generation will stay with brown (black) power technologies. So this will never officially be a sector, but obviously there are analysts (and reporters and blogs) who will cover it as though it is one.