System Design Best Practices and Considerations

A photovoltaic (PV) system is an electric system that generates electricity from the sun’s energy. A major component of a PV system is the array, which is commonly the most visible part of a PV system and is composed of the solar panels that convert light from the sun into electrical energy. The amount of energy a solar array generates is highly correlated to how directly the array faces the sun. As the sun moves through the sky, morning to night and season to season, the optimal positioning of a solar array changes. To achieve optimal positioning at all times, an array that moves with the sun (using a tool called a tracker) can be installed, but more commonly solar arrays are mounted in a fixed position, usually on rooftops at the angle that results in getting the maximum benefit of solar power relative to the location of that particular PV system.

A contractor holding a hardhat and safety glasses in front of a field of solar panels.

There are optimal ranges for fixed mounted systems but site-specific characteristics such as the angle of a roof and shading from trees and other items near the PV system’s location commonly guide the positioning of an array. Shade cast on a PV system throughout a majority of the day is not ideal and should usually be avoided.

Tilt and Azimuth – What are they and why do they matter?


When deciding if a solar array is well designed, you should consider the tilt for fixed mounted arrays.  Tilt is the angle of an array relative to flat or perfectly level. An array with a zero-degree tilt would lay flat, parallel to the ground, and an array with 90° tilt would stand on end, perpendicular to the ground. Based on an analysis using PV Watts, the ideal tilt for PV systems in Illinois is between 33-37° depending on the latitude of the array. The average tilt for systems in the Illinois Shines program is 22° and the range is between 0-90°. 


Another design consideration is azimuth, that refers to the direction which a solar array faces. True south or 180° azimuth is the ideal azimuth for a solar array in Illinois. East has an azimuth of 90°, west 270°, and north 360°.  Many well-designed systems do not face true south. In the Illinois Shines program the average azimuth is 174° and the range is between 0-360°. 

Additional Factors When Considering a Proposal

Orientation should also be considered when reviewing a solar project proposal, because other components of the proposal may make a suboptimally-oriented array worthwhile and vice-versa. Review the renewable energy credit (REC) estimate of the proposal, which refers to the amount of electricity, measured in megawatt-hours (MWh), that a PV system will generate over a given period of time. Keep in mind that one REC is equal to 1 megawatt-hour of solar energy generation. A company that is applying your PV system to the Illinois Shines program must provide you with a REC estimate. Whereas one proposed array might have an ideal orientation, another proposed array might use a slightly less ideal orientation but utilize more efficient panels and produce more RECs overall.
The Disclosure Form that is required to present to customers includes information on estimated kWh production of the proposed system, as well as other Project Design Specifications and Project Performance information. Ultimately, the REC estimate will allow you to make an “apples-to-apples” comparison between the production of two PV system proposals.

Ultimately, the  REC estimate will allow you to make an “apples-to-apples” comparison between the production of two PV system proposals.

There are a wide variety of products that make up a PV system.  When considering a PV system, you should also consider the warranties on the overall system, the inverter(s), and the PV modules.

Depending on whether you purchase your PV system outright, take out a loan to finance the system, lease the system, or enter into a PPA (Power Purchase Agreement is a contract to purchase just the energy generated by the system), some of the above system design considerations may be more or less relevant to your particular situation. In all situations, however, it’s important to evaluate the payback period for your solar PV system, weighing any upfront or ongoing costs relative to the projected savings from reduced utility bills.