Solar Panel Efficiency Explained

What is the Efficiency of Solar Panels

Solar Panel Efficiency refers to the percentage of sunlight that is converted into usable electricity under standard test conditions. The amount of solar energy which hits the surface of a photovoltaic cell or panel depends on climate and weather conditions, but the conversion efficiency of domestic solar panels has increased rapidly over the past years.

Since not all of the sun’s energy that reaches a solar panel is converted into electricity as most of it is lost. Then the conversion efficiency of a solar cell or panel, is a key parameter of any photovoltaic cell as the more efficient the solar panel is, the more electricity it will produce making it more competitive against other sources of energy generation.

Solar energy can be harnessed in one of two primary ways. Firstly, photovoltaics (PVs) are semiconductors that generate electricity directly from sunlight. Second, solar thermal technologies use sunlight to generate heat instead of converting the sunlight directly into electricity. The captured heat can then be used to heat water for domestic uses, warm buildings and work spaces, or heat fluids to drive electricity-generating turbines.

Measuring Solar Panel Efficiency

Generally, solar thermal panels are more space efficient than solar photovoltaics in collecting heat from the sun’s rays. Also, the technology required to capture and heat water is much less complex than for photovoltaics. This makes solar thermal panels an ideal solution for heating up water.

Solar panel efficiency for a photovoltaic panel is an indication of its performance in converting sunlight into electricity. The efficiency of solar panels has improved dramatically in recent years, increasing from about 15% to more than 20% using newer technologies.

High-efficiency solar panels can reach as much as 23% with the power rating of domestic photovoltaic panels increasing from 250W to 370W. But what does it mean, what is solar panel efficiency and how can we calculate the actual efficiency of a solar panel.

Types of PV Cells and How They Affect Efficiency

There are basically three types of solar photovoltaic cells available today which can be used to build solar panels: Monocrystalline, Polycrystalline, and Thin film (also known as Amorphous). Each PV cell type has its own advantages, disadvantages, and configuration.

The Different Types of Photovoltaic Cells

Monocrystalline Cells: these are formed from one single but solid crystalline silicon semiconductor, hence their name “mono”. Monocrystalline cells offer more area for electrons to flow, resulting in a smaller, more efficient solar panel. Their disadvantage is that they tend to be more expensive than other panel types.
Polycrystalline Cells: these are constructed from many individual shards of silicon semiconductor melted together and cooled very slowly under controlled conditions resulting in crystalline regions which are separated by large gaps. While not as efficient as the monocrystalline cells due to these gaps, polycrystalline cells and panels are cheaper to manufacture.
Thin-film Cells: are second generation, non-crystalline Amorphous silicon (a-Si) and Cadmium Telluride (CdTe) technologies produced by depositing ultra-thin layers of semiconductor materials onto a flexible or rigid substrate of lower cost. This makes them ideal for small-scale or portable applications. However, their disadvantages include lower solar panel efficiencies and higher degradation rates over time.

Therefore it is not easy to say which type of photovoltaic cell is the best or the worst or the more efficient. They are just different and built for different applications and budgets with about 90% of the photovoltaic panels installed are of crystalline silicon.

How to Calculate Solar Panel Efficiency

The photovoltaic conversion efficiency, which can be determined from the current versus voltage (I-V) characteristic curves of an illuminated photovoltaic cell (or panel) is an important factor in identifying its quality, performance and efficiency under varying environmental conditions.

Then the need for accurate comparisons of performance between different photovoltaic cells (or panels) has given rise to efficiency measurements performed under standard solar test conditions (STC).

Solar panel efficiency (η) is a commonly used parameter to compare the performance of one PV cell to another, or one solar panel to another. Conversion efficiency, expressed as a percentage (%), depends mainly on the intensity of the incident sunlight hitting its surface and the temperature of the solar cell. Then the efficiency of a PV cell is defined as being:

Solar Panel Efficiency Formula

Where efficiency, η (eta) is the solar panel efficiency, P_MAX is the maximum electrical power, divided by the sum of the panel area (A) in m², to the irradiance intensity (E) measured in watts-per-metre-squared (W/m2). Note that P_MAX is the maximum power output rating of the PV cell or panel at “full sun” with an irradiance of 1000 W/m2.

Photovoltaic Efficiency Example

A 2 m² panel of solar cells has a maximum power output of 400 watts. What would be its conversion efficiency value.

Thus the 2 m² panel has a conversion efficiency or 20%. That is, 20% of the sunlight hitting the panel is converted into electrical energy, while the remaining 80% is lost.

Now let’s consider the following solar panel label below from a Sunket 600W monocrystalline solar panel as another example.

The Sunket SKT-600-M12 panels label states that its dimensional size is 2172 mm by 1303 mm. Thus the actual size of the semiconductor silicon panel which receives the sunlight is: 2.172 m by 1.303 m = 2.83 m².

Therefore, we can calculate the solar panel efficiency of this 600 watt crystalline silicon photovoltaic module as being:

Then, this 600W PV panel has a calculated efficiency value of 21.2%, which is the same as the manufacture’s quoted value of 21.2% given on its official online datasheet.

We can also use the solar panel efficiency formula to calculate the expected power output of the 600 watt panel at different solar irradiance levels.

So for example, if the total irradiance falling on the previous Sunket panel is measured as being 667 W/m². What will be Sunket’s monocrystalline panels power output at this irradiance level.

Power Output of the 600W Panel at Reduced Light

Then we can see that the power output of a solar panel takes into account its efficiency. That is the m² size of the panels you install on your roof is determined by their efficiency as a panel with reduced efficiency will be physical larger in size compared to a panel of the same rated output power but with a higher efficiency. Since their electrical conversion efficiency is somewhat fixed at between 15% to 20%.

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Alternatively, if two solar panels are of the same physical m² size, the panel with the lower efficiency value will have a lower rated output power. If a photovoltaic solar panel could convert all of the light energy at 100% efficiency, then the power output of the panel would be the same as the power available. In reality this is not possible due to several factors. But what influences the efficiency of a solar panel.

Solar Panel Efficiency Factors

Photovoltaic solar panel efficiency can be affected by many different factors in many different ways. Some of these factors can be avoided, such as better positioning, reduced shading, etc.

While others are intrinsic to the installation itself such as the surrounding area as well as the spectrum and intensity of the incident sunlight and the ambient temperature of the photovoltaic cells and panel.

The main factors which impacts on solar panel efficiency and how they convert sunlight into usable electricity, depends on several factors which include:

Quality of Materials – The type and quality of the silicon semiconductor material used in the manufacture of photovoltaic cells plays a significant role in their efficiency. Photovoltaic cells are made with different types of silicon materials, such as monocrystalline, polycrystalline, and thin-film. Higher purity silicon-based PV cells allow for better electron movement, leading to higher photovoltaic efficiencies.
PV Cell Temperature – PV cell temperature is one of the most important factors responsible for lowering the efficiency and performance of PV cells. Photovoltaic solar panels generally perform better at lower temperatures since they prefer colder clear skies making them less efficient at higher temperatures. Excessive heat increases the resistance in the silicon semiconductor material, reducing the energy output. The peak wattage rating of a panel given on its name plate is determined at a STC temperature of 25^oC.
Light Intensity and Spectrum – The intensity and spectrum of light plays a significant role in determining solar panel efficiency. The higher the intensity of sunlight, the more photons are available to be absorbed by the PV cells meaning more energy can be converted into electricity, resulting in higher power output. Since sunlight is composed of a broad range of wavelengths across the electromagnetic spectrum, including visible light, infrared (IR), and ultraviolet (UV) light. The ability of PV cells to absorb and convert light into electricity depends on the bandgap of the material being used. Maximum solar irradiance across a unit area of PV surface is defined as 1000 W/m² at an Air Mass of 1.5.
Panel Orientation and Angle – The angle of incident at which the sunlight strikes the panel influences efficiency. Photovoltaic panels must be positioned to receive maximum sunlight as optimal orientation and tilt can maximize exposure to sunlight and thereby improve efficiency when the sun is directly overhead. The optimal angle depends on geographic location and the time of year. Tracking systems that follow the sun can improve efficiency by ensuring the panels are always positioned for optimal light absorption.
Quality of Components – The arrangement of the PV cells within a panel (or module) and how they are interconnected impacts efficiency. More compact layouts reduce energy losses. The quality and condition of the components used, including the photovoltaic panels, inverter (which converts the DC from the panels into AC for use in homes), and other hardware, affect overall system efficiency. High-quality components tend to be more electrically efficient and durable.
Age of the Panel – Over time, photovoltaic solar panels degrade, reducing their efficiency. PV panels are expected to operate for 20 to 30 years, but over time the conversion efficiency can degrade by as much as 0.5% to 1% per year due to the daily exposure to the elements and normal wear and tear as it heats up and cools down daily.

Then, solar panel efficiency depends on a variety of different factors, resulting from a combination of irradiance light intensity, ambient temperature, elevation angle of the sun and local weather conditions. Shading from nearby trees, buildings, or other obstacles can significantly reduce the efficiency of solar panels as well as dust, dirt, and debris on the surface of panels which can block sunlight reducing their power output.

In general, the relative effectiveness of a photovoltaic solar panel especially in terms of its peak wattage generation can depend on many factors, especially what the panel is connected to. The actual performance compared to the manufacturers rated performance can be very different, since manufacturers want to sell PV panels and systems according to the price per watt of power produced and not its photovoltaic efficiency.

Solar Panel Efficiency Summary

In summary, we have seen here that a solar panel’s energy conversion efficiency refers to the percentage of sunlight that a solar panel can convert into usable electrical energy. In simple terms, it measures how effectively the panel transforms the energy from the sun into electricity.

When sunlight (composed of photons) hits a photovoltaic cell (or panel), the PV cell absorbs the energy from the photons. This energy excites electrons in the cell, creating an electric current. The efficiency of the solar panel indicates how much of the total sunlight that strikes the panel is converted into electrical power when connected to an external electrical circuit. So collecting the solar power more effectively can increase efficiency.

The energy conversion efficiency, η (eta) is typically calculated using the previous formula. So if a solar panel receives 1,000 watts of sunlight per square meter and produces 200 watts of electricity, its efficiency would be 20%. This means that 80% of the energy that reaches the panel is not used. Note that no solar panel is 100% efficient.

The following list are the typical percentage efficiencies for each of the three different types of photovoltaic solar panels currently available:

Monocrystalline: These are typically the most efficient, with conversion rates ranging from between 18% and 22%.
Polycrystalline: Slightly less efficient, with efficiencies between 15% and 18%.
Thin-film (Amorphous): These are the least efficient, with rates between 10% and 12%, but they are lighter and more flexible.

Clearly, the monocrystalline panels with an average efficiency of between 18% and 24% is the one to use. Generally, you shouldn’t settle for anything rated below 20%, especially considering that the higher the efficiency the fewer panels you will need to produce a certain amount of free solar power.

The Importance of High Efficiency

High efficiency solar panels generate more electricity from the same surface area, which can reduce the number of panels needed to meet your energy requirements. This is especially important on roof installations or in areas where space is limited.

Photovoltaic panels with higher efficiency tend to perform better in low light conditions (like cloudy days) compared to less efficient panels. So with higher efficiency can produce more power output for the same amount of sunlight. That is Higher Efficiency = More Electricity.

To learn more about how solar panel efficiency measures how effectively a particular photovoltaic solar panel transforms the energy from the sun into electricity. Or to obtain more information about the different types of photovoltaic panels available. Then find your ideal Photovoltaic Solar Panel today direct from Amazon and understand how you can use solar panel efficiency ratings to your advantage to power your home.

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