30% ?? where would that come from? By the way, did you read the 2 part electric power piece by Calder in ProBoat? A very good piece with real numbers by a tree hugger. But it just doesn't work, just too many losses converting diese to electric, storing it, then turning it back into rotational power. Not to mention the high voltages that must be involved for it to work, and the weight and disposal problems with batterys.
If it were viable to do what you are suggesting. diesel/electric locomotives (where battery weight is no penalty) would have done done it decades ago. Generating extra power on the flats, storing it, & using it on hills.