Once you have prioritized your municipal sites for solar, you can roughly estimate the solar capacity (in kilowatts [kW]) and energy (in kilowatt-hours [kWh]) production potential by using free software such as NREL’s System Advisor Model (SAM) or PVWatts Calculator. SAM is a detailed program that determines the capacity and energy of a solar system by taking into account solar tilt, ground cover ratio, and type of PV module. PVWatts, on the other hand, is a simpler online tool that estimates the capacity and energy available from a user drawn area at a specified location. A quick rule of thumb for commercial rooftop solar is that roughly 70% of a roof can be utilized for solar panels, of which a system size is 10 W/ft2 (for 20 degree tilted systems) to 16 W/ft2 (for horizontal systems). For ground-mounted systems, five acres of land is typically needed to reach a capacity of 1 MW (4-5 W/ft2).

Next, refine each site’s system capacity based on these two important local regulations:

The refined solar energy potential of your prioritized sites will help to inform the aggregated project system size, in MW, in your RFP. The City of Pasadena’s Distributed PV Potential Assessment can serve as an example final output of a detailed solar PV assessment.