Need help finding federal funding sources for your projects? Try our new and improved Funding Tool

Procurement Guidance Menu

On-Site Solar

Estimate Solar Energy Potential for Prioritized Sites

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:

  • System Size Restrictions

    Determine your state interconnection rules, which determine how solar PV systems are connected to the grid. Also check for the solar overproduction limits set by your utility to identify the maximum allowable solar PV system size.

  • Aggregated and Net Metering Regulations

    Net metering, which credits solar customers for excess solar power fed into the grid, can have a significant impact on determining the optimal solar PV system size. Meter aggregation (follow the link and click on your state), which allows the transfer of solar credits across buildings, significantly affects solar PV sizing for aggregated solar PV projects involving multiple sites.

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.


  • Create a list of prioritized sites with their associated key data points
  • Estimate the solar energy potential of the prioritized sites