Measuring power output by modified prony brake

[Helmizar]

Hi guys

I have problem, basically I want to measure the power output from a water wheel.
so basically I measured the power output by connecting the pulley that attach on the shaft of the water wheel. then this pulley is connected to other pulley which sit on the shaft of the DC motor.
this DC motor is braking by using potentiometer as controller (which act as brake to the wheel). may be much more easy to see the drawing http://www.thescienceforum.com/images/smilies/smile.png
currently I calculate the power output by using method as shown in the attachment. but when I compared it with prony brake method, the power output is smaller than prony brake method. I get confuse now, is there any suggestion regarding my equation, probably something wrong with my calculation method
Helmizar

 

[Danger]

Could it just be due to mechanical losses in the pulley system?