The F1 cars have roughly 80kg of ballist... they said a KERS system weghts in roughly 35kg so they loose about half their ballist that they can move around the chassis. the 6sec of about 80hp is equal to about .3 sec per lap. if the driver uses the full 6 sec early in the lap he wont have any for say the back straight and will be over taken. its also strategy on the drivers part on when and when not to use it.