A guide to understanding solar output losses

Maximizing production is a common goal when investing in solar energy. Aurora Solar, a leading provider of solar design and performance software, released a guide to help understand the leading causes of energy loss in solar PV systems and how to avoid them.

Climate insurance and renewable energy risk management company KWh Analytics released its 2022 solar production index and reports that solar power resources are generally performing worse than expected. Systems installed since 2015 have exceeded expectations by 7–15 percent, with regional differences. How can this underperformance be avoided?

Aurora Solar The Ultimate Guide to Solar System Losses includes basic solar energy performance concepts such as the effect of tilt, orientation and shading on production metrics. The guide reviews how incompatible devices can cause losses, and examines the effects of incident angle transformers and module nameplate rating losses.

Tilt and direction

The angle of the panels affects the amount of solar radiation the system receives during the year. The Aurora report states that tilting the array toward the equator maximizes incident radiation, which increases production.

Utilizing the angle of incidence of the sun is also important in terms of production. The angle of incidence refers to the angle of the panel’s surface in relation to the sun’s rays. The angles of incidence affect the amount of sunlight that gets through the glass on the front side of the panel.

Aurora said these losses, measured as an angle of incidence modifier, typically range from 3% to 4.5%. The DeSoto model is used to understand the incidence angle modification effects.

Dirt or the accumulation of dust and other debris on the panel surface is a leading cause of energy loss in some areas. In areas with long dry seasons, it can lead to losses of 5%. In areas with frequent dust accumulations, it can increase this number by 1-2%, and in areas near high traffic areas, losses are usually another 1%. In areas with year-round rainfall, contamination is typically around 2%, Aurora said.

Performance parameters from the National Renewable Energy Laboratory (NREL) suggest that in the United States typical 5% fouling is common. The NREL model found that one annual cleaning in a system with 1.9% fouling would reduce the loss to about 1.5%. Two cleanings per year could reduce the average loss to 1.3 percent, and three cleanings per year would further reduce it to an average annual loss of 1.2 percent. The NREL site analysis of pollution effects can be found here.

Birds and bird droppings are another production concern. Bird droppings essentially block one or two cells and may not be washed away by rain. In modules without bypass diodes, a complete blockage of one or two cells can lead to loss of function of the entire module. Aurora advises quick manual cleaning of bird droppings.

Snow load is another mitigating factor. NREL losses calculated in the study ranging from 10-30% in fixed tilt systems. Snow factors can be difficult to accurately model on an annual basis, so Aurora recommends measuring on a monthly basis. A field study of snow loss estimates based on different tilting of the panels can be found here.

Shadowing is another very important part of system performance. Aurora compares a shaded solar cell to a clogged pipe. When the cell is shaded, the current through the entire cell string is reduced. The panels have integrated bypass diodes that allow the array to “bypass” the shaded cell, but at the cost of giving up any output that could have been collected from that cell. Stanford University’s analysis of shading effects can be found here.

Aurora suggests using module level power electronics (MLPE) or microinverters to avoid losses due to shading.

Environmental damage

Temperature coefficients are another factor to consider in performance. The temperature coefficient is measured when the percentage energy production drops for every 1 degree Celsius rise above the 25 degree Celsius (77 degree Fahrenheit) reference point.

Certain roofing materials absorb more heat than others, affecting performance. Panel angles can change temperature, and Aurora said flat panels tend to run hotter. The panel type also makes a difference. Thin film panels typically have a lower temperature coefficient than monocrystalline or polycrystalline solar panels.

Modules in systems with mismatched or long strings can lose another 0.01 to 3 percent of total production. Aurora uses a 2% assumption when modeling this loss class. Incompatible modules with tight power tolerances can result in 1% system loss.

Degradation by light occurs when the electrical properties of crystalline silicon solar cells change when exposed to light. Losses range from 0.5% to 1.5% and occur within the first few hours of exposure to a new panel.

The losses on the nameplate of the module represent the loss due to the difference in the declared power of the module compared to how it actually performs under normal test conditions. Aurora suggests that there are no losses to modern modules in this category, as they most accurately reflect standard test results.

However, some providers may state a performance range called “power tolerance”. It is typically expressed as a plus or minus percentage. For example, a 250 W panel with a power tolerance of +/- 5% can produce 237.5 W to 262.5 W.

Cable worries

Wiring losses typically cause another 2% system losses. If the project uses thicker yarns for short runs, these losses can be closer to one percent.

“Several components can cause voltage drop in circuits, including connections, fuses and resistors. Differences in cable length or size between parallel springs can also cause voltage drop,” says Aurora.

NREL study modeled link losses can add 0.5% loss. Wiring terminals and bypass diodes have physical imperfections that cause resistance, resulting in small voltage drops.

Inverter efficiency measures how efficiently DC energy is converted into alternating current energy. Inverter manufacturers provide both a maximum efficiency rating for performance under ideal conditions and a weighted efficiency rating for its performance under multiple conditions.

“It is important to look at the weighted efficiency, because the efficiency of the inverter changes according to the capacity it carries. Most inverters reach a peak load of about 20% and drop slightly when the load reaches the maximum input value,” Aurora’s report says.

Inverter cutoff often occurs in systems at the height of sunny days. When the DC output of the panels is greater than the DC power converted by the inverter, shear loss occurs. Aurora’s NEC validation report vol helps to properly size inverters.

The publicly available system performance model PVWatts uses a default value of 3% loss of system availability. Aurora said that systems with operational and maintenance or fault alert systems installed may experience only 0.5% loss of availability. Availability includes inverter shutdowns or failures, grid outages and other events that interrupt the PV system.

Thermal expansion and contraction, UV light, and damage from wind-blown particles reduce production over time. The solar panel manufacturers’ production warranty gives a conservative estimate of the deterioration of the panels over time.