Every day, the sun showers the planet with millions of times more energy that its people use. The only problem is that the energy is spread out over the entire earth's surface and thus must be harvested. Engineers are learning to capture and use some of this energy to make electricity for homes. A well-designed solar system can last for 20 years or more. It is quiet, produces no pollution, and saves the homeowner about half of their electric energy costs. Since the late 1970s, scientists and engineers have worked to develop cheaper and better solar systems. This article describes how engineers plan a residential solar system, and provides a glimpse of some of the math involved.