From WoWWiki

Energy is regenerated at a rate of 20 energy per tick. This is not modified or changed by any statistics. However, the Adrenaline Rush talent will double energy regeneration to 40 per tick during it's duration.

In certain cases of severe lag energy regeneration may sometimes operate erratically such as regenerating 44 energy per tick. Keep this in mind if you feel you're regenerating odd amounts of energy.