Idk I get the feeling that the GRAM might not make the cut, which seems to contribute a fair amount to the G2s efficiency. If google is trying to save money I would expect the battery, screen, camera, and GRAM to be the first things to go or get pared back
Then expect roughly the same battery life as an HTC One. Same battery capacity, larger screen area size to light up, but more energy efficient CPU. Also stock Android helps a little.
So in terms of battery if it doesn't get GRAM, the Nexus 5 will have 2 advantages over the HTC One: Snapdragon 800 and stock Android.
The HTC One has one advantage over the Nexus 5: smaller screen to keep back-lit.
And since "Screen" is typically at the top of my battery stats in settings, this could cancel out the two advantages the Nexus 5 has.