The Minnesota Pollution Control Agency (MPCA) regulates stormwater management in Minnesota under the Federal Nation Discharge Elimination System (NPDES) program. The MPCA issues permits covering every type of construction project, including solar panel installations. Several solar industry stakeholders and other agencies have requested guidance to clarify how stormwater regulations apply to these types of projects. This document was prepared in collaboration between these organizations and the MPCA.
Solar panels, roads and gravel surfaces are considered impervious surfaces. The NPDES Permit (Minnesota Construction Stormwater General Permit) requires that a certain volume of stormwater from impervious surfaces, called the treatment volume, must be treated prior to runoff. Solar panels are considered a disconnected impervious surface when water running off a panel is discharged to a pervious surface (e.g. turf, crop, perennial vegetation). The MPCA allows a volume credit for disconnected impervious surfaces. This volume credit is applied to the treatment volume, thus reducing the remaining amount of stormwater that must be treated.
Runoff from a connected impervious surface flows along the impervious surface until it reaches an inlet or stormwater treatment practice. An example is runoff from a road that discharges to a swale. Although the surface of solar panels is considered to be impervious, the solar panel farm as a whole qualifies as disconnected impervious when a) there is pervious surface between each panel, and b) there is pervious surface beneath each panel. As rainfall drips off the solar panel’s surface, some of it will infiltrate the pervious surfaces before it reaches an impervious surface such as a gravel path or road.
The MPCA developed a solar panel calculator, which is an Excel spreadsheet that quantifies the amount of runoff retained at a site with solar panels acting as disconnected impervious surfaces. More information on the methodology and the calculator can be found in the Minnesota Stormwater Manual.
The calculator is based on 35-year continuous simulation modeling. The modeling analysis considered different ratios of impervious to pervious area and different soil types. The analysis quantified the runoff reduction achieved from redirecting runoff from impervious to pervious surfaces. Inputs to the calculator include soil type, amount of impervious and pervious surface area, and the runoff depth from the solar panel. Runoff reductions resulting from impervious surface disconnection are subtracted from the required treatment volume. This runoff reduction is called the BMP volume credit.
Utilizing areas with soils that have high infiltration rates will achieve a larger BMP volume credit. Additionally, increasing the distance of pervious surface between each panel, or decreasing the ratio between impervious and pervious surfaces (I/P ratio), increases the BMP volume credit.
Pervious surface must be vegetation such as pollinator friendly plant mix, mowed lawn, or a native plant mix. It is recommended that the vegetation cover is 90 percent or better uniform coverage. Gravel is considered impervious, so any pathways or roads within the solar panel farm cannot be included in the pervious area.
Pollinator friendly plants include native plants that are highly attractive to pollinators such as native bees, honey bees, butterflies, moths, and hummingbirds, and are well-suited for plantings in urban and rural areas. A pollinator friendly plant mix is an option for the pervious surface underneath and around solar panels, but it is not required. Pollinator plant mixes are easy to establish in most regions in Minnesota and are being endorsed by many agencies around the state for their numerous benefits. For more information, link here and here.
No. However, optimal site conditions such as a low I/P ratio paired with soils with a high infiltration rate can reduce the infiltration volume requirement by over 90 percent. The BMP volume credit is dependent on soils at a site and the design specifications of the solar panels.
To account for the rest of the required infiltration volume, other stormwater treatment options are available, including but not limited to
There is not one best option for stormwater treatment. It is best to consider variables such as maintenance and operation costs, available area within the solar panel farm, and the geographical and topographical setting of your solar panel farm when planning for stormwater management.