Determining solar panel placement enables sales representatives and homeowners to modify a solar power system by adding or removing solar panels or arrays of solar panels, or changing module type. The user sees the corresponding solar energy production update instantly in a user interface. Determining solar panel placement includes receiving data corresponding to an installation location. A maximum solar panel design is determined based on the installation location. Energy production is determined for a solar panel on each section of a roof of an installation location, and the energy production and energy offset are dynamically displayed in real time when any solar panel or array is selected or deselected.