Understanding Solar Panel Power Ratings

Understanding solar panel data sheets and module direction are important factors to consider when investing in solar power. When shopping for a solar electric power system for your home or business, insist on a system PTC rating . A PTC rating is the efficiency rating of the module in real world conditions not in the factory. STC rated kilowatt-hours per day are not real condition numbers.  When comparing various solar electric applications, be sure you are comparing apples to apples.

STC in an acronym for “Factory Standard Test Conditions,” which is 1,000 watts per square meter solar irradiance, 1.5 Air Mass and 77 degrees Fahrenheit cell temperature. PTC is an acronym for “Photo Voltaic USA Test Conditions,” which were developed at the PV USA test site at the University of California Davis. The PTC rating represents a more real life condition of 1,000 watts per square meter solar irradiance, 1.5 Air Mass, 68 degrees Fahrenheit ambient temperature at 33 feet above ground level and wind speed of 1 meter per second.

The ambient temperature rating is a better standard than factory conditions because silicon solar cells average about 68 degrees above ambient temperature in the real world.  Cell voltage drops as temperature increases. A module’s power output in real life conditions is lower than the power measured at the factory where cell temperature is maintained at a controlled 77 degrees.

Cell voltage drops about 0.08 volts per degree Centigrade in environments that exceed 77 degrees Fahrenheit. Thus, an STC rating of 17 volts can actually become a PTC rating of 15 or 16 volts. Using Ohm’s Law, volts times amps is equal to watts, which equals power, so a reduced voltage means reduced watts.

 Solar Module Compass Direction

The amount of sun shining on the module will be determined based on the direction the system is facing. As the sun moves across the sky throughout the day, from the East in the morning to the West in the afternoon, the compass direction (orientation to the South, Southwest, East, etc.) of the module affects the cumulative energy output.

For this reason, maximum efficiency is gained when panels are installed facing south.  If the module is facing East or West, it will be exposed to less direct sunlight as the sun moves across the sky. There is no loss factor for south facing modules. For example, a southwest-facing module estimated energy output would be reduced by approximately 3%.

Below are several solar panel ratings that you should understand so you know what you are getting when you invest in a solar power system. Understanding and inquiring about solar panel ratings will ensure that you get the most out of your solar system.

Increasing Module Temperature Decreases Power (Temperature Factor)

Module operating temperature increases when placed in the sun. As the operating temperature increases, the power output decreases due to the properties of the conversion material.  Reduction of power output is true for all solar modules. The PV USA Test Condition (PTC) ratings take power reduction into consideration to a limited degree by calculating the PTC ratings based primarily on the specific module temperature characteristics. The PTC ratings are different for each module, and typically vary from approximately 87%-92% of the STC rating. A typical decrease in power output is approximately 12% for crystalline-based solar modules.  This decrease results in an STC rated 100 Watt DC solar module being PTC rated at approximately 88 Watts DC.

Minimum Warranted Power

 Minimum Warranted Power is a manufacturer rating. Essentially, manufacturers will guarantee that their panel’s actual power rating will not be less than a specified amount coming out of the box. This may have several names including peak tolerance rating, minimum warranted power, and negative tolerance rating. Regardless of the name, this means that a 200-Watt solar panel, with a negative tolerance rating of 10% (or a minimum power rating of 90%) is only warranted for 180 Watts out of the box.

Remember to look for the highest minimum warranted power rating and/or the lowest negative tolerance rating. Both are saying the same thing with different numbers.

STC vs. PTC Rating

 California and many other states require solar panel manufacturers to submit operating specifications for their products. These products are then further tested by an independent laboratory. In California, the agency approved to do this is PV USA. The results of PV USA tests are referred to as a panel’s PTC rating (PV USA Test Conditions). Therefore every panel will have both an STC rating (standard test conditions) and PTC rating. The PTC rating involves more extreme, real world test conditions, so you’ll find it to be lower than the STC rating and more accurate of how a solar panel will hold up. So the higher PTC rating, the better the panel.

Also, the state uses the PTC rating to calculate system rebates (i.e., expected performance). So if the STC rating for a panel is 175 Watts, and the PTC rating is only 160 Watts, the PTC rating of 160 will be the benchmark for your cash rebates — an important tip when figuring expenses and payback period for your solar system.

Efficiency Ratings

 This is the rating we’re all familiar with and the one that gets so much attention these days as solar panel developers and researchers struggle to create the highest efficiency solar panels. Efficiency rating is vital because the higher the efficiency, the more power you’ll get per square inch of panel surface. In other words, the higher the rating the less area you’ll have to use to achieve the same amount of power production. Be sure to review the factory printed specification sheet that lists solar module efficiency (not simply solar cell efficiency), along with the PTC and STC ratings described above.

At Sun City Solar Energy, we constantly research the solar panel market to offer our customers the highest producing modules available.  You can be assured that our products meet or exceed your expectations.

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