*Raise hand*
*lowers hand when I understand what you mean.*
I was going for the whole "storage space in is another space, there for it is more like a delayed teleportation from one space to another.
But I now see what you mean, the conservation of energy states that energy can neither be created nor destroyed, only to be converted or transferred, meaning that somewhere, the energy for moving the object from low point to high point or vice versa has to be turned into something in order to keep the balance. But since storage power is a ability common yet untapped/ unstudied, I and most of us here have nothing much to comment it on other than the fact that the object "gains" energy when the person is elevated or "loses" energy when lowered. The whole "person puts the object into the subspace and therefore the mass becomes zero, resulting the only energy change to be the person's mass, not person+object" is a dilemma as the energy (work done over time) for the object is not accounted. Unless there is a substantial difference in using the subspace at different height aka more energy usage the higher you go, then there is no way to show where the energy has gone.
In the example of "gaining" energy, when the person carries the object in subspace and climbs higher, the object gains potential energy from height and to counter that energy "gain", the person taking the item out needs to lose the exact amount? (I put a question mark to doubt myself. I doubt myself all the time)
Ps. I have a fun giggle at myself thinking of a child using the subspace to carry a wheel of cheese from shop to home, only to get tire out when he extracts the cheese. I suddenly like to give thanks to the bag of holding with no mumbo jumbo mass reduction/ nullification and skyrim/ fallout and other games having weight limits on their storage space.
I mean, even mass reduction/ nullification magic balances the book by counting in the mass of objects into mana usage so they don't hurt my head that much.
Have a thought like this though: