It depends on your electrical consumption and how much you want to reduce it. Also, you will need enough unshaded space with plenty of southern sun exposure, which sometimes can be limited by trees and adjacent buildings. A good rule of thumb is for each kw of solar installed you will get 1,150 kwh of power production per year. Also, for each kw of solar installed you will need approximately 100 square feet of space. Most homes and businesses cannot cover all their electric power needs with their roof space. If you live in the South Central PA region, you can calculate the KW of solar modules needed to provide 100% of your electric bill by dividing your annual KWh usage (found on your electric bill) by 1,150. For instance, if you use 10,000 KWh per year, you would need to install 10,000/1,150 or 8.7 KW of modules. Of course you can cover a lower percentage of you electric bill in order to help reduce the initial cost, and add on later if desired.