Estimation of Solar PV System Output

Factors Affecting Output

Standard Test Conditions

Solar modules produce DC electricity. The dc output of solar modules is rated by manufacturers under Standard Test Conditions (STC). These conditions are easily recreated in a factory, and allow for consistent comparisons of products, but need to be modified to estimate output under common outdoor operating conditions.

STC conditions are:

  • Solar cell temperature = 25 degree C;
  • Solar irradiance (intensity) = 1000 W/m2 (often referred to as peak sunlight intensity, comparable to clear summer noon time intensity); and
  • Solar spectrum as filtered by passing through 1.5 thickness of atmosphere (ASTM Standard Spectrum).

A manufacturer may rate a particular solar module output at 100 Watts of power under STC, and call the product a “100-watt solar module.” This module will often have a production tolerance of +/-5% of the rating, which means that the module can produce 95 Watts and still be called a “100-watt module.” To be conservative, it is best to use the low end of the power output spectrum as a starting point (95 Watts for a 100-watt module).

Temperature Losses

Module output power reduces as module temperature increases. When operating on a roof, a solar module will heat up substantially, reaching inner temperatures of 50 – 75 degree C. For crystalline modules, a typical temperature reduction factor recommended by the CEC is 89% or 0.89. So the “100-watt” module will typically operate at about 85 Watts (95 Watts x 0.89 = 85 Watts) in the middle of a spring or fall day, under full sunlight conditions.

Dirt and Dust Losses

Dirt and dust can accumulate on the solar module surface, blocking some of the sunlight and reducing output. India has a rainy season called monsoon and much of dry season. Although typical dirt and dust is cleaned off during rainy season, it is more realistic to estimate system output taking into account the reduction due to dust build up in the dry season. A typical annual dust reduction factor to use is 93% or 0.93. So the “100-watt module,” operating with some accumulated dust may operate on average at about 79 Watts (85 Watts x 0.93 = 79 Watts).

Module Mismatch Losses

The maximum power output of the total PV array is always less than the sum of the maximum output of the individual modules. This difference is a result of slight inconsistencies in performance from one module to the next and is called module mismatch and amounts to at least a 2% loss in system power. A reasonable reduction factor for these losses is 98% or 0.98. So the “100-watt module,” operating with some dust and module mismatch may operate on average at about 77 Watts (79 Watts x 0.98 = 77 Watts).

Wiring (Ohmic) Losses

Power is also lost to resistance in the system wiring. These losses should be kept to a minimum but it is difficult to keep these losses below 3% for the system. A reasonable reduction factor for these losses is 97% or 0.97. So the “100-watt module,” may give an output of about 75 Watts at inverter terminals after ohmic losses (77 Watts x 0.97 = 75 Watts).

DC to AC Conversion Losses (Inverter Losses)

The DC power generated by the solar module must be converted into common household AC power using an inverter. Some power is lost in the conversion process, and there are additional losses in the wires from the rooftop array down to the inverter and out to the house panel. Modern inverters commonly used in residential PV power systems have peak efficiencies of 92 – 94% indicated by their manufacturers, but these again are measured under well-controlled factory conditions. Actual field conditions usually result in overall DC-to-AC conversion efficiencies of about 88 – 92%, with 90% or 0.90 a reasonable compromise. So the “100-watt module,” may give output of about 67 Watts at the output terminals of inverter (75 Watts x 0.90 = 67 Watts).

Transformer Losses

The AC power generated by the inverter must be stepped up to transmit the power efficiently at higher voltage. Some power is lost in the transformation process in transformer. These losses are inclusive of ohmic losses in transformer winding, eddy current losses in laminations and all other losses in transformer. Efficiency of Power Transformers is up to 98%. So we can use the factor 0.98. In case of system not using transformer such as house hold roof top, should not consider the losses of transformer. So the “100-watt module,” may give output of about 65 Watts at the output terminals of transformer, if used (67 Watts x 0.98 = 65 Watts).

Sankey Diagram

Net Power at the Meter

So the “100-watt module” output, reduced by production tolerance, heat, dust, wiring, AC conversion, transformation and other losses will translate into about 65 Watts of AC power delivered to the meter panel during the middle of a clear day

(100 Watts x 0.95 ) x 0.89 x 0.93 x 0.98 x 0.97 x 0.90 x 0.98 = 65 Watts

Posted By: Rajesh Kumar Ghorla (Consultant)


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