Solar panels are made up of cells, each cell having an “active” surface area of around 60.0 cm². The cells are made up of semiconductor material that converts sunlight into electricity via the photovoltaic effect.
However, due to a phenomenon known as internal recombination, the cells can convert only a theoretical maximum of 33.16% (±0.5%) ^1 of the energy carried by photons with energy more significant than the bandgap into electricity.
For more energy to be converted into electricity, multiple cells must be connected, increasing cost and lowering efficiency.
In this article, we’ll cover some key points like:
- What is solar panel efficiency?
- How to increase solar panel efficiency.
- Why solar panels are so inefficient.
You’ll be surprised how low the efficiency of most solar panels is. Read through the article to find out more.
What is solar panel efficiency?
Solar panel efficiency is the percentage of the sun’s energy that is converted into electricity. It is the ratio of power out divided by power in. for example, a 100-watt panel with an efficiency of 16% would have an output of 16 watts.
The efficiency of the single-junction cell has limited the efficiency of solar panels.
The theoretical upper limit for high-efficiency solar cells is about 33.16% (±0.5%).
There are currently commercially available multi-junction photovoltaic cells with efficiencies over 44%.
NASA is working on a variety of technologies to achieve efficiencies greater than 100%. But in practice, most installed solar panels have an efficiency between 10 and 20%.
Why are solar panels so inefficient?
While there are many different types of solar cells available today, with efficiencies ranging from 10% to 45%, most commercially available solar cells have average efficiencies between 17-20%.
This means that if you place enough of them side by side, they will be more efficient than an array with 20% efficiency because it produces more energy given a particular area.
A fundamental limit is that no single cell will ever convert more than 33.7% of incoming photons into electricity.
This would completely unbind electrons from their nuclei – thus creating an electric current without moving any charge between electrodes. Today’s most efficient solar panels use highly purified silicon with extremely sharp crystal boundaries.
These semiconductor layers can convert 41% of all available energy in photosynthesis into electricity by simply moving charges through the material (not requiring chemical reactions as plants do).
This allows for an overall power conversion efficiency of 90%.
But even if we were to take pure crystalline silicon as our starting point, there are still many other factors involved in solar panels which prevent perfect efficiency.
Solar panels do not have the best area ratio to electrical power output – they absorb less light than a flat surface with the same area.
This is due to only a specific wavelength range absorbed by silicon and other materials used for solar cell production.
It’s also because multiple technologies must be used together to reach their maximum potential: photovoltaic cells can convert sunlight into electricity but need inverters to change direct current into alternating current, and an array of capacitors will store excess energy until it is required.
Inverter losses account for another 7% on average, meaning that at 20% efficiency, we could have an array capable of generating electricity at 97% efficiency – but this is not the case.
Solar panels are designed for maximum output during peak sun hours, which coincides with peak demand times in most places.
Thus, solar companies design their panels to maximize the amount of power you can expect to generate between 10 AM-2 PM; so, despite being able to use them all day long, you’d only receive ~80% capacity because they were designed to operate at high efficiencies during these ideal times.
Many components are also left out of cheaper products to keep costs down, such as bypass diodes, increasing panel efficiency by 2%.
Can solar panels be 100% efficient?
Solar panel efficiencies are limited by the single-junction cell. Solar panels act more like a valve for sunlight, allowing photons to enter but not allowing them to leave.
Photons with an energy larger than the bandgap are absorbed and create mobile electrons that can be used to create an electric current.
However, internal recombination occurs because there are electrons in states within the forbidden gap at about the middle of the solar spectrum; these act like tiny antennas absorbing specific wavelengths of light and sending the energy as heat rather than converting it into electricity.
For more energy to be converted into electricity, multiple cells must be connected, increasing cost and lowering efficiency. The current commercially available multi-junction photovoltaic cells have efficiencies of over 44%.
How can we increase solar panel efficiency?
NASA is working on a variety of technologies to increase solar cell efficiency. One of the most promising approaches would use multiple thin-film tandem cells stacked in layers, with each layer designed for a different part of the solar spectrum. This approach employs co-evaporation
To create semiconductor layers that have different bandgaps, ensuring that photons always encounter a material whose bandgap energy is smaller than their photon energy.
The top cell in a triple-junction cell can be tuned to capture mainly blue and green light; this cell would be several thousand angstroms thick because it needs only to absorb short-wavelength light.
Each subsequent cell absorbs progressively longer wavelengths; thus, current technology implementations consist of about 20 semiconductor layers, each a thousand angstroms or so thick.
The result is an overall thickness of about 30 to 60 micrometers – thin enough for large-scale deployment on rooftops or in power plants.
How close are we to making 100% efficient solar panels?
The closest scientists have come so far would be by using multi-junction photovoltaic cells with efficiencies over 44%.
NASA is working on various technologies to achieve efficiencies greater than 100%, but most installed solar panels have an efficiency between 10 and 20% in practice.
This means that if these panels were placed side by side, they would create more energy than the same number of panels, with an average efficiency of 20%.
Overall, the best commercially available solar panels have efficiencies between 17 and 20%.
While several promising technologies are in development, none of them will be ready for commercial use until around 2020. In the meantime, other approaches could increase efficiency without requiring any new technology.
One solution would be to add tiny nanocones or nanowires to existing cells, which would scatter light into sideways paths that would otherwise not be collected by normal flat cells.
This approach is desirable because it can be applied cheaply and efficiently at the manufacturing stage, even to old panels – and it should work with almost any type of cell.
A coating could also absorb some wavelengths better than others due to its chemical composition, requiring scientists to fine-tune the spectrum of light absorbed.
Either approach could enhance the efficiency of solar panels on rooftops by 30%. Still, it would probably first be implemented on solar farms, where the greater surface area means more significant cost savings.
How do you calculate solar panel output?
To calculate the output of a solar panel, you ideally need to know which part of the color spectrum and thus which wavelength range your cell is most sensitive to.
Some manufacturers state this explicitly in their datasheets or webpages, but if not, you can find out by testing it yourself.
A multimeter with a photovoltaic function would be sufficient for measuring VOC (open-circuit voltage) and JSC (short-circuit current). These values can then be used to calculate your power output.
As long as we know our short circuit current value (JSC) and one other value, such as open-circuit voltage (VOC), we can quickly determine how much power (AC) our panel puts out.
Once we know the AC output value, we can also calculate the percentage of light our solar panel can convert into electricity (% VOC).
This value is highly dependent on your panels’ spectral response curve, which you would have to derive yourself or find in the datasheet.
What is the most efficient solar panel?
There are a lot of factors that influence which solar panel is the most efficient one. For instance, there are different kinds of silicon cells with drastically different efficiencies:
• monocrystalline Si has efficiencies from 16 to 18%
• multi-crystalline Si has efficiencies from 13 to 15%
• thin-film technologies have achieved up to 20%, but they currently have low market penetration.
The leading companies in the solar industry are becoming more and more efficient at making silicon cells.
Industry leader SunPower recently claimed a new record with an efficiency of 24%.
Panasonic is also one to watch. It is currently developing a cell that will absorb 44% of the energy from sunlight, nearly doubling the amount of light absorbed by existing cells.
So far, there aren’t any significant manufacturers that have achieved efficiencies higher than 32%; however, their research indicates that they should reach 40% within the next five years.
What are the factors affecting solar panel efficiency?
The efficiency of a solar panel is determined by:
• the material used
• the type of cell (amorphous, monocrystalline, and polycrystalline cells)
• how it was manufactured.
Silicon is currently the most used semiconductor in solar cells; however, other materials such as gallium arsenide or cadmium telluride exist.
Silicon has to be treated with boron and phosphorus atoms to create an abundance of free electrons (electrons that can move freely through the material).
The more free electrons available, the more electricity we will get out of our solar panel. This treatment process is known as “doping” and creates either p-type or n-type silicon (positive or negative).
P-type silicon has an abundance of free “holes,” basically holes in the material filled with electrons. Holes act like positively charged particles and can move through the material just like electrons can.
