The size of your solar system will depend on a number of factors, such as how much electricity you need, the amount of hot water or heating space, the amount of light in your area, the size of your roof, and your budget. Photovoltaic systems don't have to meet all of your energy needs. In the vast majority of cases, the photovoltaic system will be connected to the grid and the resulting solar power will be sold to a power company, while you can buy electricity the usual way.