Here are the details on the topic of System Loss Assumptions
We use PVWatts as the default API for generation and losses. The system losses account for performance losses you would expect in a real system, that are not explicitly calculated by the PVWatts® model equations.
The default value for the system losses of 14% used on ENACT is based on the categories in the table below, and calculated as follows:
100% × [ 1- ( 1 - 0.02 ) × ( 1 - 0.03 ) ×
( 1 - 0.02 ) × ( 1 - 0.02 ) ×
( 1 - 0.005 ) × ( 1 - 0.015) ×
( 1- 0.01 ) × ( 1 - 0.03) ] = 14%
Please note: The Inverter's DC-to-AC Conversion Efficiency has a default value of 96% not included in the system loss percentage (separate input area) Also, PVWatts calculates temperature-related losses as a function of the cell temperature, so you should not include temperature loss in the system loss percentage.
If you want to use a value other than the default, you can adjust the total value to account for it.
Default values for the system loss categories
||Default Value (%)
Here is the description of the Categories above:
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 modules arranged in rows when modules in one row cause shadows on those in an adjacent row. The default value of 1% represents an array with little or no shading. Shading analysis tools can determine a loss percentage for shading by nearby objects.
For one-axis trackers, PVWatts® calculates self-shading losses using the Ground Coverage Ratio
you specify under Advanced Parameters
, so you should not use the shading loss to account for self-shading with the one-axis tracking option.For fixed arrays or arrays with two-axis tracking, you can use the graph below to estimate a self-shading loss percentage when you know the ground coverage ratio (GCR). The GCR is defined as the ratio of the array area to the ground or roof area occupied by the array.
Shading loss versus ground cover ratio (GCR) for different tracking options and tilt angles at a latitude of approximately 35 degrees North.
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% would 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%.