Given that it loads nearly instantly, as opposed to a new web page which needs to send off 1000 requests to pull in 0.5GB of media and ads that I didn't request

Given that it loads nearly instantly, as opposed to a new web page which needs to send off 1000 requests to pull in 0.5GB of media and ads that I didn't request -- I say it's pretty energy efficient. Just look at energy per useful data transferred (define "useful" in some sane way). That high energy efficiency is why I favor HN over many other websites.