This means that it runs off your rear gears.
I would like to create a bicycle generator to power my television at night, so I get exercise while watching my LED TV.
On one bike I have a Nuvinci N360 hub and it is a fantastic combination.
So, you can have the more powerful and robust 500W kit, and have it detuned to 250W or 350W power levels, as you require.
The problem is that it is easy to go overboard, add too much solder and thus cause your controller to burn itself out by trying to run at too high of a power level.
Forgive me if I overlooked your information concerning he use of LiFePO4 cells.