Efficiency in Solar System

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.

Cell Efficiency
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. 

Panel Efficiency

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 color of the panel protective back sheet can affect efficiency. A black back sheet 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.

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%

 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. 

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 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.

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 affect solar panel efficiency and performance

  • Cell temperature

  • Shading

  • Panel orientation

  • Location (latitude)

  • 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 affects 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 power optimizers 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 an 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.   

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, the cell temperature is 25-35°C higher than the ambient air temperature which equates to an approximately 8-14% reduction in power output. Note cell temperature can raise to 80°C or even higher when mounted on a dark-colored rooftop during very hot, windless days.