Energy Reserve that slowly depletes on its own

[supersaiyandoyle]

How should I price an energy reserve that depletes automatically on its own?
It's got the special recharge limitation so that it doesn't fill up on its own automatically.
I thought about just putting Costs ER on an always on ability, but I'm surprised there's nothing (at least in the books I've checked) in terms of limitations, specially for energy reserve or otherwise. I thought about putting Costs ER on the Energy Reserve itself, but I'm not sure that works, and it seems kind of clunky.