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Solar Panel Efficiency: Have You Checked Your Eta Lately?


Category: Solar Panels

Solar panel efficiency

The ability of a solar module to convert sunlight into electricity is module efficiency, represented by the Greek letter eta (η). If you want to impress your friends, boast about the high eta value of your solar panels. A solar cell’s energy conversion efficiency is the percentage of light converted into electricity. This article takes you through some of the considerations relating to efficiency when you are purchasing your home solar panels. But first you need a little science.

Let’s start with irradiance (you can also call it insolation, which is from the sun, not insulation, which is the stuff you put in your walls). Irradiance is the term for solar radiation striking the surface of the earth at a specific time and place. On a sunny day the total irradiance striking the earth is about 1,000 watts per square meter – it is a standard amount of energy.

The efficiency of solar panels is based on standard testing conditions (STC), under which all solar panel manufacturers must test their modules. STC specifies a temperature of 25°C (77 F), solar irradiance of 1000 W/m2 and an air mass 1.5 (AM1.5) spectrums. The STC efficiency of a 240-watt module measuring 1.65 square meters (module length x module width) is calculated as follows: 240 watts ÷ (1.65m2(module area) x 1000 W/m2) = 14.54%. So when you read what a particular module’s efficiency is, now you know what it means.  If the spec sheet doesn’t list a module’s efficiency, you can always calculate it by taking the name plate rating, dividing it by the module’s square meter area, times 1000 w/m2. In other words: STC ÷ (module area X 1000 w/m2) x 100%.

Module manufacturers want to improve the efficiency to get more output from their modules. Since the discovery of photo-electric properties of silicon, the photovoltaic (PV) industry has been measuring and improving efficiency levels of PV cells. Knowing the efficiency values of different modules will help you build the best array possible for your solar energy system.

Related Article: Consumer Guide to Solar Panels

Crystalline panels are either poly or mono

Crystalline silicon modules are commonly available in 13 – 17% efficiency levels, with the best laboratory results around 25% for monocrystalline silicon. Monocrystalline is more efficient than polycrystalline, because the inter-cellular structure is more uniform than polycrystalline – hence the electric current moves more easily. Monocrystalline panels are black, and polycrystalline are generally blue, with a faceted or textured appearance. If you look carefully, you can easily see the difference between monocrystalline and polycrystalline panels.

You will find more polycrystalline panels on the market today than monocrystalline because the mono’s are more refined and costly to manufacture. Most discount solar panels at the lower cost range are polycrystalline. Polycrystalline panels are easier to refine and fabricate into solar cells than their monocrystalline brethren.

Efficiency of photovoltaic versus thin film

When you think of solar modules, most people think of the crystalline rectangular panels. They weigh about 40 pounds per module. There is another segment of the market that is often overlooked: thin film or amorphous silicon. Thin film is much lighter than the crystalline modules, and you can roll them out onto your roof. They have less traction in the market mostly because they are less efficient than crystalline silicon.

Thin film modules are manufactured using vapor deposition techniques while crystalline silicon modules (cSi) use many of the same refining and fabrication techniques employed in the semiconductor industry. Thin film silicon typically has efficiencies of 5 – 9.5% with theoretical efficiencies of 13%. Lower efficiency levels mean installations must have larger areas to install this technology to achieve the same power generation as a more efficient crystalline array.

Related Articles:

Comparison of Thin Film and Crystalline Solar Panels

Installing a Solar Energy System

Power tolerance: results may vary

Power tolerance is the percentage of power a module may produce above (or below) the STC value. For example, a 240-watt panel with a ±5% power tolerance means that under standard test conditions the solar module you purchase could produce 12 watts below 240 watts or 12 watts above 240. In other words, a manufacturer can market a module as 240 watts with a ±5% power tolerance, and the panel you buy creates only 228 watts of power! As competition has heated up, power tolerance values have narrowed (commonly ±3%) or manufacturers are marketing positive power tolerances. If a module has a negative power tolerance then the efficiency can also be less than reported on the spec sheet. The important take away is to look for power tolerance values so you know what you’re buying.

Solar panel efficiency: STC or PTC

The specifications manufacturers advertise are based on standard testing conditions (STC). Manufacturers have to conduct their own testing under these specific conditions, which details the amount of electricity a cell will produce under “standard test conditions.” A module’s maximum power (watts), maximum voltage (Vmp), maximum current (Imp), open circuit voltage (Voc), and short circuit current (Isc) are documented under STC and listed on the spec sheet. A module’s efficiency is then calculated from STC results. Most purchaser look no further than the “nameplate” rating of a certain panel.

But there are also standards developed through an independent testing laboratory. California requires an independent rating called PV USA Test Conditions or PTC ratings. Testing a module under PTC has become essential in the US, because California has by far the largest market for solar panels in the US. PTC rating is considered a more accurate assessment of PV module output.

The PTC rating, in contrast to STC, is an independent measure of a module’s efficiency and most closely approximates real solar and climatic conditions. PTC ratings are 1,000 watts/m2, 20 degrees ºC (68ºF), and wind speeds of 1 meter per second at 10 meters above ground level. PTC ratings were established to evaluate modules outside the lab environment. They document performance in the real world – resulting in lower power ratings than STC values.  PTC ratings provide an independent assessment of solar modules and are generally lower than the STC ratings listed on a spec sheet. PTC efficiency is calculated from these reduced values. For example, a 240-watt STC-rated module may only carry 210 watt PTC rating. Therefore its PTC efficiency would be: 12.7 % vs. 14.54% STC efficiency. (210 watts ÷ (1.65m2 (module area) x 1000 W/m2) = 12.7%). This is why we reference PTC values when we compare solar panels at SolarTown.

Related Article: Comparison of Several Leading Solar Panels

STC ratings, PTC ratings, power tolerance and efficiency values will help you spot a good value for your solar energy system. More efficient panels will be more expensive, but price points continue to improve. Consider these aspects of home solar panels when planning an array.

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