It's 1200 mAh for the one in the Cam plus and, independently, 2800 mAh for the stock battery. So if you switch it in and out all day (say, you're sightseeing and you use it about 25% of the time) you'd get 4000 total. They aren't in there together, but you could use them both to extend your day.
My question all along has been, are you able to put the 2800 stock battery in the cam plus? If you want to basically use the camera all day?