There are multiple reasons. One is that elementary uses a lot more animations and transparency. However, that not the whole picture. Also debian is a lot more conservative in design of there system. However, I like the UX of elementary a lot. ;-)
Well I know you already stumbled upon powertop you might want to give this another try. Did you use the report function already?
For those who running into similar power consumption issues and don't know the tool yet, here is short description on how to install powertop:
To install the tool open a terminal (SUPER + T)
sudo apt-get install powertop
sudo powertop --calibrate
The calibration can take up to one hour. Don't use your laptop during the process to get the best results.
sudo powertop --auto-tune
Set the optimized parameters automatically. Keep in mind,these settings are not permanent. The system will reset back to “Bad” power performance after restarting the machine.
sudo powertop --html=power_report.html
powertop will create a report in the power_report.html listing the top power consumers. Maybe there is a tool that points out. Good luck!
Powertop is a really complex tool. You can take a look on the user guide here