Every day, the sun showers the Earth with millions of times more energy than its people use. The only problem is that energy is spread out over the entire Earth's surface and must be harvested. Engineers are learning to capture and use some of this energy to make electricity for homes. Solar panels make up the heart of a solar system. They can be large, and they require ongoing care and careful planning in the design phase. The panels are actually a large version of the little solar cells that power the calculators people use. This article describes the planning that goes into a residential solar system and shows some of the math used in the system's design.