I agree with PS and JT.
If approval has been properly granted to a unit owner for this use of common property then it cannot be revoked except by provisions that may vary from state to state (a special resolution in the ACT).
As a matter of principle, in spite of PS’s answer to Q.1, I would strongly argue that approval should not be given without either 1) reserving similar areas of roof for each unit that could practically use it, or 2) using an appropriate amount of roof area for a communal PV system first. This comes from the principle that common property should not be given away for the benefit of only one or a few owners if it could be used for the benefit of all.
If the OC consumes power for indoor lighting of corridors or underground parking or lifts or air-conditioning, IE daytime use, then it would benefit from a PV system sized to match its daytime consumption and thereby largely avoid paying for daytime electricity use.