I guess its because of some problem that needs engines to directly feed into their own power bank… But the EC capacity is like, 0.1 units, so it isn't much of a unbalance. But I did have some cases where I had a lot of engines on my rocket and I got +1 unit of power from the engines "batteries". Maybe someone who knows how these things function can tell us more about it, I only got my feet wet in this system.