A solar panel datasheet typically provides technical specification data, such as power, current, and voltage, under various test circumstances. It is the main aspect for comparing the performance of solar panels. Three standards of test conditions are used to measure these key parameters, each with its approach and context.
Before further reviewing the product specification, it would be better to understand in advance some of the terms and parameters used to make the decision that suits our power requirement.
Test conditions for solar panels: STC vs. PTC vs. NOCT
Standard Test Condition (STC) is a widely used industry standard for testing solar panels and their electrical properties. It is driven by the requirement for a fair comparison between manufacturers, which demands the development of a broadly applicable standard. As a side note, the cell temperature parameter is used instead of the ambient temperature in this standard. This test is conducted under ideal conditions, in which the sunlight is at its peak, the panel is perpendicular to the sun, and the ambient temperature is low enough that overheating is unlikely to occur.
Irradiance: 1000 W/m²
Air mass: 1.5
STC is mostly considered when calculating the size of the equipment and how it relates to the rated power. However, the STC rating does not represent the whole performance of a solar panel because certain performance parameters, such as temperature coefficient and sensitivity, are not well-captured. The STC rating alone also does not indicate build quality and how well it performs in real conditions.
Another set of test conditions were created to meet this requirement, known as PVUSA Test Condition (PTC). PVUSA stands for Photovoltaics for Utility-Scale Applications, which is under the National Renewable Energy Laboratory (NREL) development. In contrast to STC, PTC sets the test ambient temperature and simulates the increase in cell temperature at specific wind speeds.
Irradiance: 1000 W/m²
Wind speed: 1 m/s
Air mass: 1.5
Tested at 10 m above ground level
PTC rating will always be lower than STC rating because of the more realistic test conditions, but it can estimate performance more precisely, such as power loss events that might occur in real-time. It is due to the physics fact that if the panel heats up, the output voltage drops, and so does the power. The power loss significantly impacts performance and becomes a key feature to consider while choosing solar panels.
When cells are exposed to sunlight, their temperature might rise above the ambient temperature. As cell temperature affects performance in general, it is crucial to identify the panel operating temperature, which is difficult to quantify due to variable of environment. Therefore, a standard for measuring cell temperature should be established. The operating temperature of the solar panel cell under this standard is defined as Nominal Operating Cell Temperature (NOCT). Generally, NOCT will be approximately 20-25°C higher than the ambient temperature, with an average temperature of around 45°C.
Irradiance: 800 W/m²
Wind speed: 1 m/s
Air mass: 1.5
Although NOCT is not required for design purposes, this parameter can indicate how well a panel does under different conditions, especially thermal characteristics. When comparing two panels with the same STC rating, PTC rating and NOCT can be used as an additional comparison indicator.
What is the efficiency and temperature coefficient of a solar panel?
Module efficiency (%) is related to how much radiation absorbed can be converted to electricity at STC. For solar panels, the amount of electricity produced is a matter of efficiency value and environmental conditions that affect the amount of radiation received. Nevertheless, a more efficient panel would deliver the same amount of power while reducing the panel dimension.
Meanwhile, the temperature coefficient (%/°C) describes the percentage of peak power losses per 1°C increase in temperature from STC. Temperature coefficient and NOCT can be used to estimate power loss under operational conditions, with the following equation:
Based on this relationship, a smaller temperature coefficient and NOCT would lead to less power loss. Thus, better performance could be expected. This equation shows the performance gap of operations in the real environment compared to the ideal conditions.
Both efficiency and temperature coefficient depends on the module material, cell type, and manufacturing process. Moreover, those values are highly related to the operating temperature. The temperature coefficient, as previously stated, defines the amount of power loss proportionate to temperature rise. The power loss would imply a reduction in output power, lowering the efficiency percentage. To conclude, the temperature coefficient determines the efficiency of a panel in terms of how much electricity is lost under its operating temperature.
The major electrical terms you need to know
Output relationships of a module are represented in a graph called an "I-V curve". It refers to the value of current and voltage corresponding to the power while considering the irradiance from each specific operating condition.
Maximum Power Point (Pmax)
Pmax (W) is the maximum rated power output at STC. Because this parameter is measured under ideal conditions, a comparative parameter that is relevantly measured under operating conditions similar to the real one, like PTC, is required. Comparing the Pmax with the PTC power rating would give us a more comprehensive overview of performance.
The graph above shows that Pmax is achieved right before the voltage drops as the cell temperature increases. It is derived from the product of current and voltage at certain values known as Vmpp and Impp.
Pmax value might be lower or higher during measurement and could be different for each panel since solar cells are made in batches. A massive test is performed to measure the power output, and the panel will be sorted into a group of ranges based on it for sales segmentation. Hence, power tolerance (W or %) is defined to give more details on the deviation of Pmax. Operating current and voltage may differ significantly for the operation at each end of the tolerance range, determining the resulting power output.
Maximum Power Point Voltage (Vmpp)
Vmpp (V) is the voltage where the Pmax is achieved. It is typically listed in the solar panel specification. It depends mostly on the temperature and will drop drastically at a specific temperature threshold.
Maximum Power Point Current (Impp)
Impp (A) is the current where the Pmax is achieved. It is typically listed in the solar panel specification.
Open Circuit Voltage (Voc)
Voc (V) is the voltage in no-load condition. It represents the maximum voltage and is commonly used to define the solar panel configuration for the number of panels wired in series to the inverter/charge controller. It is important to prevent overvoltage, which could damage the equipment.
Short Circuit Current (Isc)
Isc (A) is the current in no-load condition. It represents the maximum current when the short circuit occurs by connecting the positive and negative leads of the wire. This is important as a safety concern and ensuring the needs of protective devices like fuses or breakers.
Other terms related to a panel’s performance
Wind load (Pa), also known as back maximum static load, refers to the amount of wind force that a panel can bear with. The wind force is proportional to the wind speed. In the case of extreme weather, the mounting of the panel would need special attention to guarantee system robustness.
Snow load (Pa), also known as front maximum static load, refers to pressure from a static snow weight that a panel can bear. For the area with heavy snow, it is recommended to use a stronger panel so that it won’t break easily.
STC is still the most used standard for factory testing of solar panels. If the specifications contain ratings under PTC or NOCT, it could be an additional consideration to compare the overall performance. Testing under PTC and NOCT conditions is also recommended to complete technical data on solar panel specifications. Some characteristics, such as efficiency and power loss, may be better described using test conditions similar to the operation. We also need to consider the environmental and climatic factors in addition to technical parameters since they impact the solar panel's performance and the strength of the structure required for installation.