Even though I think solar is impractical as a primary source for various reasons, it doesn't take a lot.

David MacKay in "Sustainable Energy: Without the Hot Air" did a calculation circa 2010. To fulfill the world's energy needs back then, a 10 km^2 area in the Sahara desert would be sufficient. Even if you scaled that to 100 km^2, it's absolutely tiny on a global scale, and panels have only become more efficient since then.

The challenge of course is storage and distribution, but yeah, in terms of land area, it's not much.

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.