Depends on where you live. Sure, if you an apartment heavy area like Singapore, then no - but based on some very quick googling, it looks like globally around 60% to 80% of the population live in detached or semi-detached houses, where they own the roof, so there's your free space.

Even when people rent, I can see having solar panels simply becoming factored into the rent equation. If the renter can expect to save $100 off their monthly power bill, then the landlord can easily ask for an additional $80 in rent, which means they make more profit over the owned lifetime of the property.