Solar PV rooftop systems: Four key factors affecting the output of your rooftop systemn

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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 o C; solar irradiance (intensity) = 1000 W/m (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 250 Watts of power under STC, and call the
product a “250-watt solar module.” This module will often have a production tolerance of +/-5% of the rating, which means that the module can produce 240 Watts and still be called a “250-watt module.” To be conservative, it is best to use the low end of the power output spectrum as a starting point (240 Watts for a 250-watt module).

Following are the five key factor which affect the eystem output of your rooftop system.

  1. Temperature

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 o C. For crystalline modules, a typical temperature reduction factor recommended by the CEC is 90% or 0.89. So the “250-watt” module will typically operate at about 225 Watts (250 Watts x 0.9 = 225 Watts) in the middle of a spring or fall day, under full sunlight conditions.

2. Dirt and dust

Dirt and dust can accumulate on the solar module surface, blocking some of the sunlight and reducing output. Much of California has a rainy season and a dry season. Although typical dirt and dust is cleaned off during every rainy season, it is more realistic to estimate system output taking into account the reduction due to dust buildup in the dry season. A typical annual dust reduction factor to use is 93% or 0.93. So the “250-
watt module,” operating with some accumulated dust may operate on average at about 232.5 Watts (250 Watts x 0.93 = 232.5 Watts).

3. Mismatch and wiring 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. 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 95% or 0.95.

4. Dc to ac conversion 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 “250-watt module” output, reduced by production tolerance, heat, dust, wiring, ac conversion, and other losses will translate into about 168 Watts of AC power delivered to the house panel during the middle of a clear day (250 Watts x 0.95 x 0.89 x 0.93 x 0.95 x 0.90 = 168 Watts).

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