……… and typically for NSW, the not so simple answer is only after ≥75% of those present both personally and by proxy at a General Meeting vote YES, and with that percentage being determined from the total lot unit entitlement of those voting that way, divided by the aggregate unit entitlement for the Plan.
That’s a Special Resolution under the provisions of Sect 65A of the NSW Strata Schemes Management Act (1996) authorising the Owners Corporation (O/C) to add or erect a new structure (solar panels) on its Common Property.
KWP’s right about any feed-in tariff or expenditure off-set from gross or net metering being regarded as income, and the tidiest way of management that is for the O/C to hold the solar system in trust for all Owners, as in that way the annual income (in whatever form) can be apportioned to each in accordance with their lot’s units of entitlement, and be declared on the Income Tax Return of each Owner as may be appropriate.