How could anyone possibly care about such a thing?
The graph linked[1] does not show that much of a difference between different browsers - the difference appears exaggerated because the scale on the Y-axis does not start at zero. Thus, it is very likely that whatever power saving that you will derive are negligible and don't make a difference in the grand scheme of things.
[1] - http://tctechcrunch2011.files.wordpress.com/2013/06/browser_...