In Windows 8, powercfg command line utility was expanded with a new useful tool called Sleep Study. it generates a relatively easy to understand detailed html report that shows what software and drivers consumed most of your battery while the device was in Connected Standby state. 

First off all you need to make sure that your device supports Connected Standby (now actually called InstantGo).
Open elevated command prompt and run: powercfg /a
If your device supports connected standby (InstantGo) you will find Standby (Connected) under Available Sleep States.

To perform actual Sleep Study, run: powercfg /sleepstudy -duration 3
-duration 3 specify how many days include in the report.

The report will be saved to the current folder. If you want to save to your desktop, precede the above command with: cd “c:\users\<username>\desktop”.
<username> is your Windows user name.