After the building design is finalized, there will be a specific area (the ‘solar zone’) that hosts the maximum number of modules or laminates that can fit on the roof. Issues that will be considered and reviewed include:
- code setbacks (e.g. prescription by federal, state, or local code authorities requiring egress for firefighting or setback from roof edges/corners to protect from wind uplift effects);
- rooftop equipment (e.g. a 1.2-m [4-ft] HVAC unit means space loss);
- roof access for maintenance and fire-fighting;
- desired array size; and
- space loss due to wire management.
Calculating ROI
Attention to all the details in designing a solar system is crucial to achieve the most power production and to run an accurate return on investment (ROI) for the system. Sophisticated software currently available enables accurate prediction of the expenses, computing all project-specific variables including maintenance, sun-hours, system efficiency, energy cost, and cost escalation. With a basic ROI, the owner system designer and owner can accurately evaluate whether the proposition makes financial sense (Figure 4).
For example, if the net system cost after a tax credit is $236,250 (original net investment was $337,500), and the PVs become ‘cash-positive’ in their sixth year, the ROI is the positive cash flow for the balance of the service life. In this particular hypothetical case, the payback at the end of 25 years (i.e. warranted period for solar modules) is $1,230,377. However, modules will typically have a longer service life than their warranty—at the end of 30 years, the ROI would be $1,585,377. If the system is still producing power after 35 years, the return on investment would be $1,940,377. This sounds like a really good deal, but it is very important to remember the costs of roof replacement are not being factored in.
The roof, after all, is the solar mounting platform. This means beyond knowing the life expectancy of the PV panels, it is absolutely critical to know the ramifications of the roofing assembly type, its service life, replacement costs, and whether it was included in the financial proforma.
