This would depend on the total surface area of roofs in a city.

VOA: special.2010.03.02

It has a surface area of almost twenty thousand square kilometers.

VOA: special.2009.08.19

Plate tectonics is the area of science that explains why the surface of the Earth changes and how those changes cause earthquakes.

VOA: special.2010.01.12

They made several trips from the landing area to study the surface of the moon.

VOA: special.2009.07.29

With such a vehicle, astronauts could explore a much larger area of the moon's surface.

VOA: special.2009.07.29