I like the current system where power regen blocks use block dimensions to give a bonus, since it requires shipbuilders to consider the placement of blocks more, instead of just making large solid masses of regen (which you have also expressed dislike for). This system would be incompatible with your proposed change, since power capacity operates only on size rather than block dimensions.
A solution might be to make each group have its own sliders, as with weapons, but this has a separate problem: it will be a hassle to configure if you have more than a few power structures.
Your system has the option of either being implemented in a way that encourages lazy shipbuilding or in a way that is inconvenient to anyone with more than a few connected power structures.