You're definitely not going to power all of NYC by rooftop solar, but also look at the size of the warehouse roofs (empty, no solar) in NJ just across the river, and out on long island. And Staten island. Etc.
Empty land is not necessarily a requirement.
Parking lot per-row shade structures that integrate PV panels on top are also a thing now, and there are sure a shitload of parking lots in NJ and out on long island.
I'm not from the area, but isn't Manhattan well known for its tall buildings? How would those buildings' shade affect solar panels closer to ground level, e.g. warehouse roofs and parking shade structures?
Rooftop solar is far more expensive than solar put on land. The cost of rooftop solar (particularly homeowner solar) is usually hidden by implementing a big subsidy to wealthy consumers that is paid for by less wealthy consumers - sort of a reverse robin hood scheme.
We are well past the point where anyone can argue we should subsidize rooftop solar because we need to help build a fledgling industry - yet I see solar advocates continue to advocate for this wealth transfer from the poor to the well off.
Rooftops are unused space, but the land use is generally a small part of the cost. Utility scale solar has real economies of scale compared to one-off small installations on someone's roof.
