[suicideneil]

Current, unless Im mistaken. The components are rated for a certain amount of voltage and current (amps), so all things being equal there will be a maximum amount of power (watts) you can generate before the current climbs too high (assuming you dont go over the rated voltage)- this could be because of excessive gearing or load causing the motor to pull too much current. Thats why its always best to use higher voltage and lower kv motors to generate the maximum possible/ allowable amount of power, as it keeps the current draw down resulting in lower running temps and greater run time.