Add the most power where it's best suited - at the crank. We've been very impressed with these BAFANG center drives. The HD is a different beast all together BIGGER and more  Robust, easy to install, comfortable and intuitive to use. We've chosen this setup for some of our Alfine hub-gear equipped bikes and they cope well. The trick is to back pedal 1/4 of a turn when shifting gear, this reduces to zero the strain on the gears. Our setup comes with 480Wh (12ah 576WH ) battery which is 40-50 km range depending on rider effort and weight.We also have 40-60 ah options for the ultra long range addicts.
 Do drop by one of our stores for a test run- we're sure you'll leave impressed.  
We have limited availability of 40Ah and 60 Ah Samsung cell packs ...mad punch for mega drives :)
It's important to understand this conversion kit can be programmed to sustain 65 km/h assisted bicycle riding - so the law prohibits us to install or sell these kits for on the road use.