I'd bet that your average 1000 MW coal or nuclear station has a bigger footprint than 1 square km. But those darned "sun angle", "nighttime", and "cloudy day" issues...those are tough indeed to overcome.
Here are some numbers on land use for different energy sources, based on a 1000 MWe design:
nuclear: 1-10 km2
solar: 20-50 km2
wind: 50-150 km2
biomass: 4000-6000 km2
Obviously, some of those are extrapolated based on scale-up of smaller systems, since we don't have any 1000 MWe solar or biomass power plants out there.
But those darned "sun angle", "nighttime", and "cloudy day" issues...those are tough indeed to overcome.
Not to mention the one that I always bring up to the sun worshippers that I have yet to hear a satisfactory answer for, that pesky old natural phenomenon called night.
Kilometers? We don't need no stinkin' kilometers!