Solar Panel Efficiency
Solar panel efficiency is a measure of the amount of solar energy (irradiation) which falls on a panel surface and is converted into electricity. Due to many recent advances in solar cell technology, the average panel conversion efficiency has increased from 15% to almost 20%. This large jump in efficiency resulted in the average power of a standard size solar panel to increase from 250W up to 320W.
As explained below, panel efficiency is determined by two main factors; the photovoltaic (PV) cell efficiency, based on the cell design and silicon type, and the total panel efficiency, based on the cell layout, configuration and panel size.
The cell efficiency is determined by the cell structure and base silicon material used which generally either P-type or N-type. Cell efficiency is calculated by what is known as the fill factor (FF) which is the maximum conversion efficiency of a PV cell at the optimum voltage and current.
The cell design plays a significant role in panel efficiency. Key characteristics include silicon type, wafer size, number of busbars and finger layout. The high-cost IBC cells are currently the most efficient (20-22%), due to the high purity N-type silicon cell base and no losses from busbar/finger shading. However, recent mono PERC cells and the latest heterojunction (HJT) cells have also achieved efficiency levels above 20%.
Total Panel efficiency is measured under standard test conditions (STC), based on a cell temperature of 25°C, solar irradiance of 1000W/m2 and Air Mass of 1.5. The efficiency value is calculated by the output power rating divided by the total panel area. Efficiency can be further influenced by several factors including, cell efficiency, the distance between the cells, and the interconnection of the cells.
Surprisingly, even the colour of the panel protective backsheet can affect efficiency. A black backsheet might look more aesthetically pleasing, but it absorbs more heat and increases cell temperature, which in turn slightly reduces total conversion efficiency.
Panels built using advanced IBC cells are the most efficient, followed the half-cut and multi busbar monocrystalline cells, mono shingled cells and finally standard 60 cell (4-5 busbar) mono cells. Common 60 cell poly or polycrystalline panels generally the least efficient and lowest cost panels.
Top 5 most efficient solar panels
Here are the top 5 most efficient solar panel manufacturers available in 2019. In general the most efficient panels use premium grade N-type IBC silicon cells manufactured by SunPower and LG, however these panels also come at a premium price. Besides REC which also use N-type silicon cells, most other manufacturers use the lower cost P-type monocrystalline PERC cells in either a standard 60 cell or half-cut 120 cell format.
SunPower - 22.6%
LG - 21.6%
REC - 19.8%
Qcells - 19.8%
Winaico - 19.4%
See the complete list of the top 10 most efficient and popular solar panels currently available from the worlds leading manufacturers towards the end of this article.
Note the efficiency listed on the solar panel specification sheet and quoted by the panel manufacturer should be the solar panel efficiency, not the cell efficiency which will be higher.
Why efficiency matters
The term efficiency is thrown around a lot but a slightly more efficient panel doesn’t always equate to a better quality panel. Many people consider efficiency to be the most important criteria when selecting a solar panel but what matters most is the manufacturing quality which is related to real world performance, reliability, manufacturers service, and warranty conditions. Read more about selecting the best quality solar panels here.
Solar panel efficiency does generally give a good indication of longer term performance especially as many high efficiency panels use higher grade silicon cells with improved temperature coefficient, performance and lower degradation over time. Some manufacturers such as LG and SunPower even offer warranties with 88% or more retained power output after 25 years of use.
Area Vs Efficiency
Efficiency does make a big difference in the amount of roof area required. Higher efficiency panels generate more energy per square meter and thus require less overall area. This is perfect when roof space is limited and can also allow larger capacity systems to be fitted to any roof. For example 12 x high efficiency 360W solar panels such as those from LG or SunPower with a 21.2% conversion efficiency will provide up to 1100W (1.1kW) more total solar capacity than the same number and size 270W panels with a lower 16.5% efficiency.
12 x 270W panels at 16.5% efficiency = 3.2kW
12 x 360W panels at 21.2% efficiency = 4.3kW
Real world efficiency
In real world use the panel operating efficiency is dependent on a number of external factors listed below which can add up to greatly reduce both panel and overall system performance:
Factor which effect solar panel efficiency and performance
Time of year
Dust and dirt
The two factors which have the biggest impact on panel efficiency in real world use are cell temperature and shading. Of course if a panel is fully shaded the power output will be close to zero, but partial shading can also have a big impact not only on panel efficiency but whole system efficiency. For example slight shading can reduce panel power output by 50% or more which in turn can reduce the entire string power by 20-30%. Strings of panels are connected in series and shading one panel effects the whole string. Therefore it is very important to try to reduce or eliminate shading if possible. Luckily there are special add-on devices known as optimisers and micro-inverters which can reduce the negative effect of shading on the whole string especially when only one or two panels are shaded.
The power temperature coefficient
The power output of a solar panel rated in Watts (W) is performed at standard test conditions (STC) and measured at a cell temperature of 25°C. However in real world use cell temperature generally rises well above 25°C depending on the ambient air temperature, time of day and amount of solar irradiance (solar energy - W/m2).
Generally the cell temperature is 25-35°C higher than ambient air temperature which equates to approximately 8-14% reduction in total power output depending on the type of solar cell and its temperature coefficient.
The rising cell temperature reduces power output by a specific amount for every degree above 25°C. This is known as the power temperature coefficient which is measured in % / °C. Monocrystalline cells have an average temperature coefficient (or loss) of -0.38% /°C while polycrystalline cells are slightly higher at -0.41% /°C. Monocrystalline IBC cells have a much better (lower) temperature coefficient of around -0.30% /°C while by far the best performing cells at high temperatures are the HJC cells which are as low as -0.26% /°C.
Temperature coefficient comparison
Power temperature coefficient is measured in % per °C - Lower is more efficient
Polycrystalline cells - 0.4 to 0.43 % /°C
Monocrystalline cells - 0.37 to 0.40 % /°C
Monocrystalline IBC cells - 0.29 to 0.31 % /°C
Monocrystalline HJC cells - 0.26 to 0.27 % /°C
Generally cell temperature is 25-35°C higher than the ambient air temperature which equates to approximately 8-14% reduction in power output. Note cell temperature can raise to 80°C or even higher when mounted on a dark coloured rooftop during very hot, windless days.
Efficiency of different solar cell types
Polycrystalline - 15 to 18%
Monocrystalline - 16.5 to 19%
Polycrystalline PERC - 17 to 19.5%
Monocrystalline PERC - 17.5 to 20%
Monocrystalline N-type - 19.5 to 20.5%
Monocrystalline N-type HJC - 19 to 21%
Monocrystalline N-type IBC - 20 to 22%
most efficient solar panels - Top 10
By far the most efficient solar panels available in 2019 still use the premium high purity N-type IBC cells manufactured by SunPower and LG , however these panels also come at a premium price. As highlighted in the table below most other manufacturers have moved from poly and standard mono to the more efficient mono PERC and half-cut mono PERC cells.
The most efficient solar panels from the top 10 leading manufacturers*
SunPower - Maxeon 3 400W - 22.6%
LG - Neon R 370W - 21.4%
REC - N-Peak Series 330W - 19.8%
Qcells - Q.Peak Duo 330W - 19.8%
Winaico - WSP-MX 335W - 19.4%
Jinko Solar - Eagle 60M 325W - 19.3%
JA Solar - JAM60S03 325W - 19.3%
Longi Solar - LR6-60HPH 320W - 19.3%
Phono Solar - Twin plus 320W - 19.3%
Trina Solar - Honey M plus 315W - 19.2%
* Based on standard 60 (120) or 96 cell residential panels available in March 2019, models and specifications may vary depending on country.
Cost Vs Efficiency
All manufacturers produce a range of panels with different efficiency ratings depending on the silicon type used and whether they incorporate PERC, multi busbar or other cell technologies. Very efficient panels above 20% are generally much more expensive, so if cost is a major limitation it would be better suited to locations with limited mounting space, otherwise you can pay a premium for the same power capacity which could be achieved by using 1 or 2 additional panels. However high efficiency panels using N-type cells are very high quality and will almost always outperform and outlast lower efficiency panels due to lower degradation rates, so the extra cost is usually well worth it in the long term.
Example a high efficiency 330W panel could cost $300 or more while a common 285W panel typically cost closer to $180. With both made by the same manufacturer, this equates to roughly $0.60 per Watt compared to $0.90 per Watt. Although in the case the leading manufacturers such as LG, Sunpower, Winaico and REC the more expensive panels have higher performance, lower degradation rates and generally come with a longer 'manufacturer' warranty period, so it is often a very wise investment.
Panel Size Vs Efficiency
Panel size is not directly related to efficiency as the larger panels generally use more cells to achieve greater power output. The two most common size panels use 60 cells or 72 cells, however a few manufacturers like SunPower produce 96 and 104 cell panels using smaller 5” cells. Most residential rooftop installations use the standard 6” square 60 cell panels while commercial systems and large scale solar farms mostly use the larger 72 cell panels. In North America many residential and commercial systems use the high efficiency 96 or 104 cell panels from SunPower or Panasonic.
60 cell panel : Approx width 1.0m x length 1.65m
72 cell panel : Approx width 1.0m x length 2.0m
96 cell panel: Approx width 1.05m x length 1.60m
A standard size 60 cell (1m x 1.6m) panel with 17-19% efficiency typically has a power rating of 290-310 Watts where as a panel with higher efficiency of the same size can produce up to 330W. As previously explained the most efficient panels use the high performance N-type IBC or Interdigitated Back Contact cells which can achieve up to 22% panel efficiency and generate an impressive 360 to 400 Watts.
The new generation half-cut or split cell modules have double the amount of cells at roughly the same size. A panel with 60 cells in half-cell format is doubled to 120, and 72 cells in half-cell format is 144 cells. The half cut cell configuration is slightly more efficient as the panel voltage is still roughly the same but the current is split between the to halves.
120 cell panel : Approx width 1.0m x length 1.65m
144 cell panel : Approx width 1.0m x length 2.0m