I'm guessing it's less than the one or two dozen 60 watt bulbs I used to use to light my house. But yes, I've heard this for a long time - at my work we used twinax or fiber often for top of rack switching and that's one of the reasons I was told. Not sure if twinax is much less power intensive, but I do have a buddy who's using regular old coax in his house (moca or something like that) as a multi-gig backbone (still less than 10G) now and he likes it. Nowadays that I have more money, I would have run fiber alongside the cat 5/6 cable to future proof things, but I'm told terminating fiber is a technical challenge. I guess there's no free or even cheap lunch for 10G yet.