The significance of using a lightweight high pressure composite accumulator in a hydraulic hybrid vehicle is best explained by the Ragone chart. Conventional batteries may provide excellent energy storage capabilities for the desirable driving range of smaller electric and electric hybrid vehicles. However they lack the ability to provide enough power (energy drawn instantly) to help propel heavier vehicles for a given mass. Accumulators and specifically lightweight composite ones provide the potent specific power so desirable for propelling these medium and heavy duty trucks and vehicles. It is the high specific power density of these accumulators that allows efficient harnessing of the energy stored by regenerative braking.