For me opera uses less resources/battery than chrome or firefox on my macbook air (2013). I'm using opera 28 though.
My suggestions for best resource usage/performance:
Make sure you are using ppapi flash (http://get.adobe.com/flashplayer/otherversions/), and disable npapi flash in opera:plugins
Enable click to play for plugins: Opera menu > settings > websites > click to play
Enable tab hibernation: opera://flags/#tab-hibernation
If you are using adblock or adblock plus, consider switching to ublock (much lighter on memory and cpu): https://addons.opera.com/en/extensions/details/ublock/?display=en