What is problem with battery life of XPS15 just purchased? October 2016

Custom built and delivered yesterday. My new XPS15, which I made sure to turn on with full battery charge today says battery 78% depleted already, after only 3 hours of operation.  Most of that time on BATTERY SAVER.  I too, am a bit concerned about the gap between advertised battery life and reality.  I will let you know when it is fully depleted and how long it really lasted.