Solar System Size
It depends on how much electricity you use. Two homes the exact same size will use different amounts of electricity and therefore require different size systems. Major electrical loads within a home are air conditioners, electric heaters, pools, and other heating elements or motors.
That being said, an average 2,500 sq. ft. single-family home uses about 5,000 kilowatt-hours of electricity per year. A 3 kilowatt (AC) system will generate nearly all the electricity required for such a home on an annual basis. Sometimes a PV system can be installed that is slightly over-sized to allow for the possibility of a growing family or other factors that may play a part. When a system overproduces electricity, the unused portion is fed back into the grid and will actually be credited back to your utility account in a program called net-metering.