i would think that the weight of the batteries involved is the problem and the time spent "hot recharging" could be an issue aswell.
your 120w / kg battery would have the same performance as the engine in my car if it weighed 1000kg's
also im pretty sure 120watts per hour is 2 watts per minute if you want it to last for the hour. (120w for 1 minute)
i dont know if i am correct but im sure somone does
