I was curious about this number, so: 10 km^2 is 10mil square meters, Googling suggests that the theoretical maximum energy captured by a square meter of solar panel is well under 0.5 kW, so well under 12 kWh per day. Say 10 kWh for neatness. Then multiplying by 10mil gives 100mil kWh. More Googling suggests that 10 TWh is a comfortable lower bound for daily world energy usage, but 100mil kWh is 0.1 TWh.

So maybe 1000 km^2 is more like right order of magnitude. That's still tiny, about Hong Kong-sized. Even 100000 km^2 is about South Korea.

I'm guessing it was supposed to be (10 km)^2, not 10 km^2.