Let me answer that with an image.

What's being depicted here, is how much of the Earth's surface would have to be covered in solar panels to fuel the world's electricity needs. That image comes from a graduate thesis from a college student in Germany. That large square titled "Welt" is all we would need to power the human race, and it's about 158×158 miles in size. Obviously, making a 25,000 square mile solar farm wouldn't be practical in real life, but it illustrates a point.