SOLAR RESOURCE DATA

System Losses Categories

The default system losses percentage is appropriate for most typical photovoltaic systems. If you want to change the value, you can either type a different value, or use the system losses calculator to calculate a value based on the system losses percentage categories described below.

Soiling

Losses due to dirt and other foreign matter on the surface of the PV module that prevent solar radiation from reaching the cells. Soiling is location- and weather-dependent. There are greater soiling losses in high-traffic, high-pollution areas with infrequent rain. For northern locations, snow reduces the energy produced, depending on the amount of snow and how long it remains on the PV modules. NREL continues to work on improving the modeling of soiling and snow and is working to include historical datasets as available.

Shading

Reduction in the incident solar radiation from shadows caused by objects near the array such as buildings or trees, or by self-shading for fixed arrays or arrays with two-axis tracking. PVWatts^® calculates self-shading losses for one-axis trackers, so you should not use the shading loss to account for self-shading with the one-axis tracking option. The default value of 1% represents an array with no shading. Shading analysis tools can determine a loss percentage for shading by nearby objects. For fixed arrays or arrays with two-axis tracking that consist of multiple rows of modules, you can use the table below to choose a loss percentage to represent self-shading (losses that occur when modules in one row cause shadows on those in an adjacent row). The graph shows the shading derate factor as a function of ground cover ratio (GCR) for one- and two-axis tracking, and for fixed arrays with tilt angles ranging from 0 to 40 degrees from horizontal. To convert the derate factor to a percentage, subtract it from one and multiply it by 100%. The GCR is defined as the ratio of the array area to the ground or roof area occupied by the array. The graph shows that for a given tracking option or tilt angle, a derate factor closer to 1 (less shading loss) corresponds to more space between rows (smaller GCR). For fixed arrays, you can reduce the area required by the array by using a smaller tilt angle with less row spacing (larger GCR) to achieve the same shading derate factor. For a given shading derate factor, two-axis tracking requires the most ground area compared to one-axis tracking or a fixed array. If you know the GCR value for your array, you can use the graph to estimate the appropriate shading derate factor. Industry practice is to optimize the use of space by configuring the PV system for a GCR that corresponds to a shading derate factor of 0.975 (2.5% loss).



Shading derate factor versus ground cover ratio (GCR) for different tracking options and tilt angles

Snow

Reduction in the system's annual output due to snow covering the array. The default value is zero, assuming either that there is never snow on the array, or that the array is kept clear of snow.

Mismatch

Electrical losses due to slight differences caused by manufacturing imperfections between modules in the array that cause the modules to have slightly different current-voltage characteristics. The default value of is 2%.

Wiring

Resistive losses in the DC and AC wires connecting modules, inverters, and other parts of the system. The default value is 2%.

Connections

Resistive losses in electrical connectors in the system. The default value is 0.5%.

Light-Induced Degradation

Effect of the reduction in the array's power during the first few months of its operation caused by light-induced degradation of photovoltaic cells. The default value is 1.5%.

Nameplate Rating

The nameplate rating loss accounts for the accuracy of the manufacturer's nameplate rating. Field measurements of the electrical characteristics of photovoltaic modules in the array may show that they differ from their nameplate rating. A nameplate rating loss of 5% indicates that testing yielded power measurements at Standard Test Conditions (STC) that were 5% less than the manufacturer's nameplate rating. The default value is 1%.

Age

Effect of weathering of the photovoltaic modules on the array's performance over time. The default value is zero.

Availability

Reduction in the system's output cause by scheduled and unscheduled system shutdown for maintenance, grid outages, and other operational factors. The default value is 3%.