The value watts/square foot (W/ft2) is a value of "energy
density" for solar panels, indicating their peak power output.
Mass-produced solar panels are currently producing up to 13 W/ft2
or about 140 W/m2.
The SI unit watts/square meter (W/m2) is also the unit of solar
irradiance, which measures the energy received by a surface exposed
to sunlight. The average solar radiation received at Earth's
surface is about 342 W/m2 and the maximum is about 1000 W/m2.