Enter roof size and panel size to estimate how many panels can fit and the system size. Assumes 75% usable roof area for setbacks, vents, and spacing. 5 feet long, occupying an area of roughly 17. When looking into a system for your home, the amount of. . This passage explains how to calculate solar panel needs, including total and usable roof area, module sizes, and effective installation factors. Your yearly electricity use, the required system capacity, and site factors such as panel efficiency, roof pitch, orientation, and shading all determine the space needed. South-facing roof sections typically receive 5-6 peak sun hours daily in North America making them ideal for panel placement. For example, a panel with 20% efficiency will turn 20% of the sunlight it captures into power, while the remaining 80% is lost due to heat or reflection. Higher efficiency means: How Is. .