The average American household uses about 11,000 kW-hrs a year, according to the first site I found, which if I've done the math right is about 1300 Watts.

If you're calculating how many you need to power a city you don't need to know the average usage, you need to know the peak.
But I assume these are designed to be in addition to the existing power grid with the owner selling spare power to the grid.