True, but this doesn't really work for densely populated areas. There isn't enough roof space on top of a 20-story apartment or office building to place enough solar panels to serve the building's needs.

For places like Barcelona:

New York:

Seoul:

etc. there's a lot of energy demand, but all of the nearby ground space is already occupied. Even if you put solar panels on top of all the buildings, each rooftop wouldn't be enough to power its own building, so collectively you would only get a fraction of the city's energy needs. The cost of doing each install and the wiring infrastructure would outweigh the benefit, it would never be practical.

*Edit: just to ballpark this, New York City used 15-16 billion kWh in Jan 2026, so ~15 million MWh/month, 180 million MWh/year. The Mojave Solar Project is one of the largest solar installations in the world. It generates ~580 GWh/year (580,000 MWh/year). So, to serve New York City we need only 310 equivalent MSP installations. The MSP installation takes up ~1765 acres, so we only need about 540,000 acres (2100 sq km), or a little over 1/10 of the state of New Jersey.

Just for New York City. Not the whole state.

And that's assuming reliable output, with no transmission losses.

And that estimate is probably too low, because any solar installation in that area wouldn't get the same amount of regular sunlight as the Mojave Desert.