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By solar panel efficiency we refer to the rate at which a photovoltaic panel converts solar energy into electrical energy. In general, a typical efficiency level of PV panels ranges between 12-16%, though recent technological improvements suggest we will soon be talking of efficiencies in the range well above 20%!

Solar panel efficiency

Solar panel efficiency is a measure of the solar cell’s ability to convert the solar energy to which it is exposed to into useful electrical power. The solar panel’s energy conversion efficiency is expressed as a percentage of the cell’s output power (watts) over the input sunlight energy (irradiance in W/m2) and the surface area of the solar panel (in m2). Considering a solar panel with a surface area of 1 meter sq. (m2) and with solar panel efficiency of 20%, at standard test conditions, i.e. amongst other conditions at clear weather with irradiance of 1000 W/ m2 and temperature of 25 °C, it will produce an output of 200 watts.

Consequently, the solar panel used in our illustration above, will produce more power than the power output of (STC) on a clear day with the sun high in the sky and less power on a cloudy day or when the sun is low.

Given a constant rated power for two different solar panels, e.g. 250 watts, their efficiency level will determine their surface area respectively. For example, a solar panel with efficiency 10% will have twice the surface area of a panel with efficiency 20% given they have the same rated output wattage – see what size of solar panels for home

solar panel efficiency PV cell

Factors affecting solar panel efficiency

From above definitions, it can be easily understood that Solar panel efficiency is influenced by many factors. The most significant are gathered and presented below:

Types of solar panels

  • Monocrystalline silicone photovoltaic cells, or single crystalline solar panels have been considered to be more efficient primarily because they have been found to exhibit a higher peak efficiency; consequently, monocrystalline solar panels were historically used for residential installations where installation area is limited. However, the statement that monocrystalline panels are more efficient than polycrystalline solar panels is rather controversial and subject to manufacturers’ specifications.
  • Polycrystalline solar panels have, up to now, been considered less efficient when compared to monocrystalline sola panels. However, given latest technological evolvement, polycrystalline silicon has managed to cover this gap, if not to surpass the performance of monocrystallic, especially at temperatures higher than STC.
  • A third type of solar panel is thin-film solar panels which are less efficient than crystalline silicon cells but also cost less. Being less efficient implies that they need a lot more surface area than crystalline silicon based cells, but they are highly flexible, thin and can be installed onto many different surfaces especially on buildings (commercial or residential solar panels – see Solar panels for green buildings and Best research-Cell Efficiencies (NREL)

Temperature

Depending on where you live, temperature may become a significant source of solar panel efficiency deterioration, especially if you live in a hot climate. The performance of solar panels may drop significantly at temperatures above (STC). One practical way of combating this is to use any means of ventilating installed solar panels to make them more efficient.

Tilt and orientation

To maximise effective exposure of the solar panel to sunlight requires that solar panels are faced to true South (for location sites within the North hemisphere) and vice versa. The inclination angle depends on the season and latitude of the site’s location; to increase exposure of solar panels to sunlight we can adjust our solar panels orientation 2 or 4 times a year according the season. Alternatively we can use a solar tracker, for maximised results, though trackers are mainly used to commercial application and not for residential applications, primarily due to higher costs and town planning restrictions.

Shades

Undoubtedly, shading will greatly affect the output performance of a solar panel. IT is important to note that when solar panels are connected in a module with one single inverter, the maximum module output is determined by the minimum performing cell; thus, in case of shade falling on a particular panel, it will influence the whole row circuit of solar panels connected together. That is why, especially in residential applications where shadings are more likely to occur, it is important to examine installation site, e.g. the roof of the building, and note any sources of shade in order to design the solar panel system accordingly. One possible solution to avoiding this solar panel bottleneck is with the use of micro-inverters; in any case, if you are planning to install solar panels for your home, it is advisable to ask for a proper installation design from potential solar panel installers in your area – you may want to see solar panels for home.

Humidity

By exposing solar panels to sunlight, they are also exposed to all nature’s conditions, including rain and humidity. If humidity manages to penetrate into the solar panel frame, photovoltaic performance will be reduced significantly and might lead to permanent deterioration of the modules performance.

Lifetime and age

Manufacturers always quote expected efficiency levels of their solar panels across their life span. For example, a common quoted is manufacturer’s performance warranty of minimum efficiency within 10 first years to be above 90% of quoted solar panel efficiency and the respective figure between 10 and 20 years around 80% or 85%. A Typical degradation rate is 0.5% per year of use.

Cleaning and maintenance

Solar panels usually require minimum maintenance as they do not incorporate mechanical moving parts; however, because they operate on sunlight passes through glass to reach the solar cell, sunlight quality is of ultimate importance and thus cleaning of the solar panel, especially the panel glass, is very crucial. As solar panels are exposed to natural conditions they gather dirt, dust, bird droppings, etc. which lead to reduction of the effective sunlight reaching the solar cells thus reducing solar panel efficiency and generated output. In cleaning roof mounted residential panels it is always advisable to seek some professional solar panel cleaning advice as any misconduct in doing so may lead to scratching the photovoltaic glass and creating a permanent more serious problem. Usually, solar panels are cleaned with lukewarm light soaped water solution and a soft non-abrasive cloth.

Monitoring performance

Keeping an eye on solar panel performance and efficiency levels through real time on-line platforms is extremely important and highly beneficial. Having access to this continues real-time data will ensure high quality control and security over the photovoltaic investment and subsequently safeguarding energy production. Deterioration or sudden decreases in photovoltaic performance are spotted immediately, thus providing early warning to act proactively and effectively against threats. On line monitoring can provide useful data for deciding on solar panel cleaning frequency, solar panel orientation corrections, and on any sources of malfunction from an early stage.

The importance of solar panel efficiency

Simply put, solar panel efficiency influences how much electricity a solar panels system is generating; this means generating more or less electricity given a constant area of solar panels, i.e. space limitations on a roof, and constant solar panel wattage (solar panel capacity). The factors influencing the output of your solar panel installation have been outlined and consist of exogenous, such as weather and location conditions, as well as endogenous sources to your solar panels, such as sola panel efficiency – you may want to refer to solar panels properties in solar energy pros and cons. Irrespective of the solar irradiation potential at your area, obtaining the highest possible output, given your budget for solar panel investment, is a primal requirement. Solar panel efficiency directly influences your output yield in generated electricity (e.g. kwh/year) and consequently influences generated income from rebates or FITs for on-grid installations. Eventually, solar panel efficiency is a primal factor influencing financial viability of your solar panels investment and the end repayment of your solar panels cost. Apart from solar panel efficiency it is crucially important to note that we are always interested in the efficieny and output performance of a solar photovoltaic system as a whole. Efficiency of solar panels, on a single panel level, is only one variable in the solar system. Learn how efficiency of your solar panel system can be affected and how it can be optimised by shoosing the appropriate type of solar inverter in micro-inverter vs string inverter.

Resources:

http://www.nrel.gov/ncpv/

cell efficiency records (NREL)

http://www.nrel.gov/ncpv/images/efficiency_chart.jpg

http://www.nrel.gov/docs/fy12osti/51664.pdf

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