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 Estimating Solar PV Potential:
NREL’s PVWatts online tool and System Advisor Model can assess the solar energy potential of rooftops as well as neighboring parking lots and land areas.
. 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 rooftop solar is that roughly 70% of a roof can be utilized for solar panels, of which it takes about 100 ft
2 (using flat panels) to 150 ft2 (for panels in tilted racks) to reach a capacity of 1 kW. For ground-mounted systems, five acres of land is typically needed to reach a capacity of 1 MW (220 ft2 per kW).

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

  • System size restrictions

    Determine your state interconnection rules (follow this link and click on “individual system capacity” in the dropdown menu), 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 System Size Limits:
    Freeing the Grid’s website lists the interconnection and overproduction limits for solar PV sizing in each state.

  • 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, which allows the transfer of solar credits across buildings, significantly affects solar PV sizing for aggregated solar PV projects involving multiple sites Aggregate and Net Metering Regulations:
    The National Conference of State Legislatures’ (NCSL’s) Aggregate Net Metering website identifies states that have meter aggregation policies.

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 City On-Site PV Assessment Example:
The City of Pasadena’s Distributed PV Potential Assessment is an example output from a third-party assessment.


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