Five factors which will affect solar system output

PV systems produce power in proportion to the intensity of sunlight striking the solar array surface. Solar system produce electricity in the proportion to the intensity of sunlight striking the solar array surface. Every day the intensity of sunshine on a surface varies, so the output of solar PV system will vary substantially from location to location. There are several different factors which will affect solar power system output.

These factors need to be understood so that the homeowner has realistic expectations of overall system output and economic benefits under variable weather conditions over time.

First factor: Standard Test Conditions

Solar PV panel produce dc electricity. The dc output of solar panel 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 oC; 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 solar PV manufacturer may rate a particular solar panel output at 300 Watts of power under STC, and call the product a “300-watt solar module.” This module will often have a production tolerance of +/-3% of the rating, which means that the module can produce 290 Watts and still be called a “300-watt module.” Therefore, it is better to use the low end of the power output spectrum as a starting point (290 Watts for a 300-watt module).

Second factor: Testing and real temperature

Solar panel output power will decrees as module temperature increases. A regular solar panel will heat up to almost 70-80 degree when operating on a roof during summery time. 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.

Third factor: Dust and Dirt of panel surface

Dust and Dirt on the solar panel surface will be accumulated over time and then block some of the sunlight which will reduce power output. Many states will have raining season and dry season, such as California in west coast or New Jersey in east coast. 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 “100- watt module,” operating with some accumulated dust may operate on average at about 79 Watts (85 Watts x 0.93 = 79 Watts).

Fourth Factor: Solar panel design mismatch and cable/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.

Fifth Factor: Power conversion losses from DC to AC

Solar inverter will convert DC power generated by the solar module to AC power. Part of solar power will be 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.

Most of inverters commonly used in residential PV power systems have peak efficiencies of 94-98% 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 “300-watt module” output, reduced by production tolerance, heat, dust, wiring, ac conversion, and other losses will translate into about 268 Watts of AC power delivered to the house panel during the middle of a clear day.

All in all, to understand several factors which will affect the output of solar panel will help customer to estimate system size more accurately.