Limitation value for a location-bound Energy Reserve?

I've a character in mind who's a wizard with a 'place of power', a workshop or sanctuary where they hang out to do ritual magic and enchanting.

I'd like to buy an Energy Reserve that's only usable within a certain area the size of a small room, only switchable with some substantial amount of preparation- say eight hours' worth of work. What would be an appropriate discount for that Availability limitation?

Limitation value for a location-bound Energy Reserve?