The first constraint of the system design is how many photovoltaic modules should be placed in the marked unobstructed area. In some cases, the roof is significantly larger than the area required by the photovoltaic array. However, when there are many smaller parts or protruding objects (such as chimneys) on the roof, it is necessary to calculate the effective area suitable for the photovoltaic system. In the process of photovoltaic array design and module selection, the effective area is extremely important.
In order to determine the effective area, the following steps are required:
1) Measure the effective space of the roof. It can be measured with a tape measure or obtained directly from the structural drawing of the building. (Note: This site defines the distance between the eaves gutter (or bottom edge) and the roof ridge as “width, and the horizontal distance along the eaves as “length”.)
2) Determine the “edge area”. Depending on the selected installation method, a border or “edge area” should be left around the eaves to ensure that the installation and future maintenance work can be carried out safely. Local regulations generally include regulations on the size of the “marginal area”. For example, in Australia, it is recommended to select 20% of the effective area for the “marginal area”.
3) Determine the maximum number of photovoltaic modules that can be installed in the effective area. The actual number of photovoltaic modules that can be installed on the roof is limited by the comparison results of the length and width of the modules and the effective space length and width of the roof. The shape of photovoltaic modules is generally rectangular, with two fixed dimensions: length and width. The modules can be installed in landscape style (long side parallel to the long side of the roof) or portrait style (long side parallel to the wide side of the roof), as shown in Figure 1 and Figure 2.
4) If space is an issue, the orientation of the most photovoltaic modules that can be installed must be calculated. If the roof is a non-rectangular area, the roof must be divided into several rectangles, and then use the following method to calculate.

① Photovoltaic modules are traditionally installed in a portrait style, and the mounting rails are installed across the roof. First, calculate the number of rows of modules that can be installed by dividing the width of the roof by the length of the module. Before the calculation, the edge area must be considered, and the edge area is subtracted from the roof width.
Then, subtract the edge area from the roof length and divide by the module width to calculate the number of module columns.
Finally, multiply the number of rows and columns of modules to get the maximum number of photovoltaic modules suitable for the roof area

②Many installation systems can now be installed in landscape painting style. First, calculate the number of rows of modules that can be installed by dividing the width of the roof by the width of the modules. Remember to subtract the edge area from the roof width.
The length of the roof minus the edge area, and then divide the length of the roof by the length of the module to get the number of module columns. Then, multiply the number of components along the length and width directions to get the total number of components.

5) Although the effective roof space is an important factor in considering the total number of photovoltaic modules, there are some other constraints. The final total number of components also depends on the choice of inverter and the number of components that can be connected in series and parallel to the inverter.