Technical Article

How Is Solar Panel Efficiency Measured?

March 21, 2023 by Chetan Kumbhar

In this technical article, we'll dive into the details of how to accurately measure solar panel efficiency using industry-standard methods.

In modern times, the demand for energy is increasing rapidly, and non-renewable sources alone cannot meet this demand. Renewables such as solar energy have gained importance due to their abundance and potential to meet the energy demand. This shift from non-renewable to renewable sources is imperative to combat the global challenges of climate change and pollution. Solar energy is one such renewable source that is gaining traction amongst researchers and the utility industry.

 

Photovoltaic System. Image used courtesy of Pixabay

 

Solar energy is abundantly available, and its primary source is the sun. Solar panels have been used for a while now and are composed of photovoltaic (PV) cells that convert solar energy into electricity. The increasing adoption of solar energy is attributed to its potential to address the challenges posed by climate change and pollution. Technical advancements in solar technology have led to the development of efficient solar panels that can generate more electricity from the same amount of sunlight.

As the world moves toward a sustainable future, the demand for renewable energy is expected to rise. The utility industry is expected to play a crucial role in this transition by investing in renewable energy sources, especially solar. The potential of solar energy is immense, and its usage can significantly reduce the carbon footprint of the energy sector.

 

What Is the Difference Between Solar Cells and Solar Panels?

What Is a Solar Cell?

A PV cell, commonly called a solar cell, is an electronic device designed to harness the energy from photons of light and convert it into electrical energy. This conversion process is achieved through the photovoltaic effect, whereby the solar cell generates an electric current when exposed to light.

 

What Is a Solar Panel?

A PV module, commonly called a solar panel, is an assemblage of PV cells integrated into a sturdy framework. Due to the limited amount of energy a single solar cell can produce, solar panels comprise several interconnected solar cells in parallel circuits to create a solar module. The size of a solar panel can range from a single module to multiple modules, depending on the extent of coverage required to harness solar energy. Figure 1 illustrates the difference between solar cells and solar panels.

 

Figure 1. Difference between solar cells and solar panels. Image used courtesy of EETech

 

Understanding Solar Cell and Solar Panel Efficiency

What Is Solar Cell Efficiency?

Solar cell efficiency measures how well a solar cell converts sunlight into usable electrical energy and is a percentage of the total amount of energy from sunlight converted into electrical energy by the solar cell. Solar cell efficiency is determined by a simple formula:

\[Efficiency(\eta)=\frac{Output\,electric\,energy\,per\,second}{Incident\,Light\,energy\,per\,second}\times100\%\]

\[\eta=\frac{P_{max}}{P_{in}}=\frac{I_{sc}\times V_{oc}\times FF}{P_{in}}\]

As per the laws of physics, there exists a theoretical maximum limit for the efficiency of photovoltaic cells, which is referred to as the Shockley-Queisser limit. This limit stands at 33.7% for photovoltaic cells composed of ideal materials. Although it is possible to exceed this limit under lab conditions, it is a challenging task for manufacturers. Achieving such high efficiency comes at a higher cost and is typically reserved for specialized applications. For instance, high-efficiency solar cells find application in advanced systems such as satellites. Despite the challenges involved in surpassing the Shockley-Queisser limit, it remains an important benchmark for the development of more efficient photovoltaic cells.

 

What Are the Parameters of Solar Cell Efficiency?

Solar cell efficiency is determined by several parameters, including Isc, Voc, Im, Vm, and FF. Isc, or short circuit current, is the current that flows through the circuit when no voltage is present. It is influenced by factors such as cell technology, the cell's surface area, and the amount of solar radiation. On the other hand, Voc, or open circuit voltage, is the voltage that is observed when no current flows through the circuit. It is dependent on cell technology and temperature.

Im and Vm are, respectively, the maximum current and maximum voltage generated by the solar cell and are crucial indicators of the cell's efficiency.

FF, or fill factor, is an essential metric for evaluating the quality of the cell. A higher FF signifies better cell quality and is generally within the range of 0.8 to 0.9. Conversely, a fill factor value below this range indicates a low-quality solar cell. FF can be determined by the equation:

\[FF=\frac{P_{max}}{I_{sc}\times V_{oc}}=\frac{I_{m}\times V_{m}}{I_{sc}\times V_{oc}}\]

 

How to Calculate Irradiance 

The following equation calculates irradiance (Pin):

\[I_{i}=I_{t}cos(\theta)\]

Where

Ii: Irradiance absorbed by the surface

It: Total Irradiance

θ : Incident Angle

Solar irradiance on a surface is greatest when the incidence angle between the sun's rays and the surface is 90°. As the angle changes, irradiance decreases, with no irradiance when the angle is 0°. NASA's solar constant value is 1,370 W/m2, which is used to determine solar values. However, at sea level, the maximum solar irradiance on a 1m2 surface is measured at around 1 kW/m2 on a clear day around solar noon at the equator. This information is essential for solar energy applications as it determines energy generation from sunlight.

 

How to Calculate Maximum Power

Pmax, also referred to as maximum power point, denotes the highest power output that a solar panel can generate under standard test conditions (STC). It is commonly known as the nameplate capacity of the panel, which is typically denoted in watts (W). 

For instance, if a solar panel is tested under STC and produces an electrical power output of 150 W, then the Pmax rating of that panel is 150 W. The Pmax rating serves as a crucial metric in determining the performance and efficiency of a solar panel. The Pmax rating of a residential solar panel may vary between 275 and 400 W on average, depending on the specific panel design and manufacturing process.

 

How to Calculate Solar Panel Efficiency

To estimate the efficiency of a solar panel, expressed as a percentage, use the following calculation:

\[Efficiency=\frac{Panel\,Power(in\,kW)}{Panel\,Length\times Panel\,Width(in\,m)}\times100\%\]

The efficacy of a solar panel is significantly influenced by its surface area, which determines its ability to convert sunlight into electrical energy. Evaluating the efficiency of a solar panel involves a comparative analysis of the solar energy received from the sun and the resulting electricity produced by the panel. It is impractical for a solar panel to transform all of the incident light energy into electricity, as this would imply a 100% efficiency rate.

 

Examples

To understand how to calculate solar panel efficiency using 300 W monocrystalline silicon cells, the specification of monocrystalline silicon cells is as follows:

 

Table 1. Specification for a monocrystalline silicon cell

Dimensions 

16499 3.5 cm

Peak Power (Pmax)

0.3kW

Maximum Power Voltage (Vm)

32.70 V

Maximum Power Current (Im)

9.17 A

Open circuit Voltage (VOC)

39.80 V

Short Circuit Current (ISC)

9.78 A

Weight

19 kg

 

From Table 1, panel power is 0.3 kW, panel length is 1.64 m, and panel width is 0.99 m. The efficiency can be calculated by:

\[Efficiency=\frac{Panel\,Power(in\,kW)}{Panel\,Length\times Panel\,Width(in\,m)}\times100\%\]

\[\eta=\frac{0.3}{1.64\times0.99}\times100\%=18.5\%\]
\[\eta=18.5\%\]

Therefore, solar panel efficiency is 18.5%.

Considering the same specification as mentioned in Table 1, the fill factor can be calculated using the formula: 

\[FF=\frac{P_{max}}{I_{sc}\times V_{oc}}=\frac{I_{m}\times V_{m}}{I_{sc}\times V_{oc}}=\frac{9.17\times 32.70}{9.78\times39.80}=0.77\]

Calculating solar cell efficiency: 

\[\eta=\frac{P_{max}}{P_{in}}=\frac{I_{sc}\times V_{oc}\times FF}{P_{in}}=\frac{9.78\times39.80\times0.77}{1000}\times100\%\]

Considering Pin as 1000 W/m2

\[\eta= 29.98%\]

Therefore, solar cell efficiency is 29.98%.

 

Factors Affecting Solar Panel Efficiency

Sunlight Intensity and Incidence Angle

Solar intensity refers to the solar energy or radiation that reaches the Earth's surface, which depends on the angle of incidence between the sun's rays and the Earth's surface. This angle varies based on geographic location, time of year, and time of day. When the sun's rays hit the Earth's surface at a 90-degree angle, they produce the most intense impact. However, in most situations, the angle of incidence is lower, resulting in a larger surface area being covered by the sun's rays and reducing the intensity of solar radiation. For instance, at a 45-degree angle of incidence, solar radiation is about 30% weaker than at a 90-degree angle due to the dispersion of solar energy over a larger surface area.

 

Temperature

Research shows photovoltaic cells perform better at lower temperatures due to a shift in semiconductor properties at higher temperatures, leading to decreased voltage despite the increased current. Extreme temperatures can also damage the cell's material, reducing its lifespan. Since a significant portion of absorbed sunlight is converted into heat, thermal management techniques can improve the efficiency and lifespan of photovoltaic cells.

 

Type of Solar Panel Technology

There are three primary categories of solar photovoltaic cells: polycrystalline, monocrystalline, thin-film, and passivated emitter and rear. Following are the distinctions between these groups and their respective efficacy levels.

  • Monocrystalline Solar Panels: These are constructed from a single, unadulterated silicon crystal sliced into multiple wafers. The dark black hue of these panels is an identifiable characteristic resulting from their utilization of pure silicon. Monocrystalline panels are the most efficient for space utilization and longevity.
  • Polycrystalline Solar Panels: These are made by melting and pouring silicon fragments from multiple crystals into a square mold, resulting in lower purity and efficiency than monocrystalline panels. They are more affordable due to minimal wastage but have a lower tolerance to high temperatures and are less effective in energy conversion and space utilization.
  • Passivated Emitter and Rear Cell (PERC) Panels: PERCs improve upon traditional monocrystalline cells by adding a passivation layer to the rear surface. This layer reflects light into the cell, reduces electron recombination, and prevents higher wavelengths from heating the cell. PERC panels offer greater energy collection in limited spaces and are only slightly more expensive to produce than traditional panels, but their efficiency can lead to a lower average cost per watt.
  • Thin-film Solar Panels: These are flexible and don't require a frame backing, making them lighter and easier to install than crystalline silicon panels. They can be customized to fit specific needs but are less efficient. Unlike silicon panels with standardized sizes, thin-film panels have varying sizes.

 

Reflectance Efficiency

In solar cells, reflectance efficiency is determined by the amount of sunlight reflected instead of absorbed. Silicon reflectance can be as high as 38%, but most solar cells have a micro-pyramid shape to reduce reflectance to about 11%. Advanced techniques like anti-reflective coatings and stacking micro- and nano-sized arrays on larger structures can further reduce reflectance to 1-2%.

 

Thermodynamic Efficiency

Thermodynamic efficiency represents the maximum possible solar cell energy conversion efficiency, around 86%. Solar cells can only generate electricity up to this point, after which excess energy turns into heat. One method to enhance efficiency is using multi-junction or tandem solar cells, dividing the solar spectrum into smaller segments to increase efficiency for each section.

 

Measuring Solar Panel Efficiency

This section explains the different methods for measuring solar panel efficiency.

 

Standard Test Conditions 

There are three conditions for solar panels:

Cell temperature =  25℃

Solar irradiance = 1000 W/m2

Air mass = 1.5

To measure solar panel efficiency under STC, follow these steps:

1. Set up a testing apparatus that can measure the voltage and current output of the solar panel under test.

2. Ensure the solar panel is exposed to a light source with an irradiance level of 1000 W/m². This can be achieved by using a solar simulator, which simulates the spectral and intensity characteristics of sunlight.

3. Keep the temperature of the solar cell constant at 25°C using a temperature-controlled chamber or heat sink.

4. Measure the voltage and current output of the solar panel at this irradiance level and cell temperature. From these measurements, calculate the power output of the solar panel.

5. Calculate the solar panel efficiency using the following formula:

\[\eta_{mp,STC}=\frac{Y_{PV}}{A_{PV}\times G_{T, STC}}\]

Where

  • \(\eta_{mp,STC}\) = the PV module's efficiency under STC [%]
  • YPV   = the PV module's rated power output under STC [kW]
  • APV = the surface area of the PV module [m2]
  • GT, STC    =  the radiation under STC [1 kW/m2

6. Repeat this process for different solar panels to compare their efficiencies under the same STC.

Solar panel efficiency can vary significantly depending on the conditions in which it is used. For example, the efficiency may decrease if the cell temperature rises above 25°C or the irradiance level is lower than 1000 W/m². Therefore, it is important to consider the actual operating conditions when selecting a solar panel for a particular application.

 

Temperature Coefficient

The temperature coefficient (TC) signifies the alteration in the power output of a solar panel when operating at temperatures other than the standard test condition temperature of 25℃. In regions with scorching weather, solar cell temperatures can escalate to over 70℃, which can substantially impact the solar panel's energy output. Suppose a solar panel has a peak power rating of 200 W at standard test conditions and a temperature coefficient of -0.5%/℃. In that case, the actual energy production of the panel would be approximately 155 W when operating at 70℃. It is worth noting that solar panels with lower temperature coefficients will perform better in high-temperature environments.

Solar panel efficiency can be calculated using the following formula:

\[Efficiency=\frac{Panel\,Power(in\,kW)}{Panel\,Length\times Panel\,Width(in\,m)}\times100\%\]

To incorporate the impact of temperature on the power output of the solar panel, the TC must be used to adjust the panel's power output for the actual temperature. Here are the steps to calculate the efficiency of a solar panel using the temperature coefficient:

1. Determine the solar panel's maximum power rating at STC in watts.

2. Find the TC of the solar panel. The temperature coefficient is expressed as a percentage change in power output per degree Celsius change in temperature.

3. Measure the actual operating temperature of the solar panel.

4. Calculate the panel's power output adjustment using the following formula:

5. Power Adjustment = Maximum Power Rating Temperature coefficient (Actual Operating Temperature - STC Temperature)

6. Calculate the actual power output of the solar panel by subtracting the power adjustment from the maximum power rating.

Determine the total solar energy input by multiplying the incident solar irradiance by the panel area.

Calculate the efficiency of the solar panel using the formula stated earlier.

By incorporating the TC in the solar panel efficiency calculation, it is possible to determine the panel's actual efficiency under real-world operating conditions.

 

Performance Ratio 

Performance ratio (PR) is commonly used to measure the efficiency of a solar panel or a solar power system. PR is the ratio of the actual energy output of the solar panel or system to its expected energy output under STC over a given period.

The expected energy output under STC is calculated based on the rated power of the solar panel, the irradiance level, and the temperature. The actual energy output is measured using a monitoring system, which records the energy generated by the solar panel or system over a certain period.

To calculate the PR of a solar panel or system, use the following formula:

\[PR=\frac{Actual\,Energy\,Output}{Rated\,Power\times Iddadiance\times Temperature}\times 100\]

The PR value ranges from 0 to 100, with higher values indicating better performance. A PR value of 100 means that the solar panel or system produces the expected energy output under STC, while a PR value of fewer than 100 means that the solar panel or system is underperforming.

PR is a useful metric for comparing the performance of different solar panels or systems, as it considers the effect of environmental factors such as temperature and irradiance. It is also useful for identifying and diagnosing issues with the solar panel or system, as a lower PR value can indicate a problem with the system's components or installation.

Overall, using PR to measure solar panel efficiency is a common and effective approach that can provide valuable insights into the performance of solar power systems.

 

Takeaways of Solar Panel Efficiency

Measuring solar panel efficiency is crucial in assessing a solar panel's performance and suitability for intended applications. Solar panel efficiency can be determined by considering various parameters, including the panel's maximum power rating and surface area. Additionally, factors such as open-circuit voltage, short-circuit current, maximum power output, and fill factor can aid in understanding the efficiency of individual solar cells. Sunlight intensity, angle of incidence, temperature, solar panel technology, reflectance efficiency, and thermodynamic efficiency are other factors that must be considered in comprehending their impact on solar panel efficiency.

Several methods exist for measuring solar panel efficiency, including standard test conditions, temperature coefficient, and performance ratio. Regular monitoring of solar panel efficiency is essential to track and ensure optimal performance and maximum energy output. Considering these factors, solar panel efficiency can be evaluated and optimized for specific requirements.

Understanding solar panel efficiency and the various factors that affect it is crucial in the selection and performance evaluation of solar panels. Using appropriate measurement techniques and monitoring procedures can help achieve maximum energy production and improve the overall efficiency of solar panels.