Solar Array Efficiency

In every solar system there are different inefficiencies because of the electrical components of the system. In every electrical component, including the panels, connections, wiring, and inverters, there are certain losses that take place. The following information is to help you understand the losses within our systems specifically, but these same factors will effect every solar system. When calculating the inefficiencies within a specific system it is called the "derate factor" and it is used to determine what percentage of losses there are from DC (Direct Current) energy to AC (Alternative Current) energy, or what you will actually be able to sell to the grid.

The overall DC to AC derate factor is calculated by multiplying the component derate factors. For the EthoSolar default values:

The value of 0.84 means that the AC power rating at STC (Standard Testing Conditions) is 84% of the nameplate DC power rating. In the case of a EthoSolar system in the first year of production, the overall default value of 0.84 will provide a reasonable estimate for modeling the energy production.

The derate factor for the PV module nameplate DC rating accounts for the accuracy of the manufacturer's nameplate rating. Field measurements of a representative sample of PV modules may show that the PV module powers are different than the nameplate rating or that they experienced light-induced degradation upon exposure (even crystalline silicon PV modules typically lose 2% of their initial power before power stabilizes after the first few hours of exposure to sunlight). A derate factor of 0.98 represents that Canadian Solar testing yielded power measurements at STC that were 2% less than the manufacturer's nameplate rating.

The derate factor for the inverter and transformer is their combined efficiency in converting DC power to AC power. CSI Inverters hold a California Efficiency Certification of 96% which conforms to the strictest testing criteria available. A list of inverter efficiencies by manufacturer is listed at HERE. These inverter efficiencies include transformer related losses when a transformer is used or required by the manufacturer.

The derate factor for PV module mismatch accounts for manufacturing tolerances that yield PV modules with slightly different current-voltage characteristics. Consequently, when connected together electrically they do not operate at their respective peak efficiencies. The default value of 0.995 represents a very small % of loss due to EthoSolar having a well engineered and designed system.

The derate factor of .995 for diodes and connections accounts for losses from voltage drops across diodes used to block the reverse flow of current and from resistive losses in electrical connections.

The derate factor of .98 for DC wiring accounts for resistive losses in the wiring between modules and the wiring connecting the PV array to the inverter. DC lines account for more losses then AC lines and therefore it is very important to locate the inverters as close to the system as possible.

The derate factor of .99 for AC wiring accounts for resistive losses in the wiring between the inverter and the connection to the local utility service. This will increase the greater the distance between your system inverters and the local utility connection point.

The derate factor of .95 for soiling accounts for dirt, snow, or other foreign matter on the front surface of the PV module that reduces the amount of solar radiation reaching the solar cells of the PV module. Dirt accumulation on the PV module surface is location and weather dependent, with greater soiling losses for high-trafffic, high-pollution areas with infrequent rain. For northern locations in winter, snow will reduce the amount of energy produced, with the severity of the reduction depending on the amount of snow received and how long it remains on the PV modules. Snow remains the longest when sub-freezing temperatures prevail, small PV array tilt angles prevent snow from sliding off, the PV array is closely integrated into the roof, and the roof or other structure in the vicinity facilitates snow drifting onto the PV modules. Roof mount systems in general will have a greater derate factor due to greater soiling and less access for cleaning.

The derate factor of .98 for system availability accounts for times when the system is off due to maintenance and inverter and utility outages. The default value of 0.98 represents the system being off for 2% of the year.

The derate factor for shading accounts for situations when PV modules are shaded by nearby buildings, objects, or other PV modules and array structure. For the default value of 1.00, EthoSolar assumes the PV modules are not shaded. Tools such as Solar Pathfinder may be used to determine a derate factor for shading by buildings and objects.

The derate factor for sun-tracking accounts for losses for one- and two-axis tracking systems when the tracking mechanisms do not keep the PV arrays at the optimum orientation with respect to the sun's position. For the default value of 1.00, EthoSolar assumes that the PV arrays of tracking systems are always positioned at their optimum orientation and performance is not adversely affected.

The derate factor for age accounts for losses in performance over time due primarily to weathering of the PV modules. The loss in performance is typically 1% per year. For the default value of 1.00, EthoSolar assumes that the PV system is in its 1st year of operation. For the 11th year of operation, a derate factor of 0.90 would be appropriate.

Overall DC to AC derate factor = 0.98 x 0.96 x 0.995 x 0.995 x 0.98 x 0.99 x 0.95 x 0.98 x 1.00 x 1.00 x 1.00 = 0.84