Get the Eagle Tree eLogger and the LCD display which will give you pack voltage and current as well as total mAhr used. I also use the optical rpm sensor and two temperature sensors for the motor and the ESC. I also record throttle position.

I leave the display in the airplane so I end up using it all the time. How often do you guys use your watt meter? I've used mine about five times in five years. It is a pain.

One of the most important things you want to do to prolong the life of your expensive batteries is to make sure you don't take off with a set that isn't fully charged and then run them too low. One glance at the display before the flight solves this problem. You also don't want to discharge them too deep and one look at the display after the flight answers that question. Maximum current and minimum voltage readings after the flight will assure that everything is working normally or tell you that something needs fixing. In short, monitoring the system will protect your investment and I believe increase your confidence in your equipment so you can concentrate on flying.

Jim O