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Efficiency of solar panels and what it depends on

When it comes to an electric generator, the most important question is how efficient it is, how much electricity or heat you can get out of it at the lowest cost. While in the past solar panels weren’t productive enough to justify their price, today it's a reliable, flexible, environmentally-friendly and affordable way to get energy from the sky daily with adequate performance. In this article we’ll focus on efficiency of solar panels: what it is, how efficient panels can be and what it depends on.

Efficiency of solar panels varies around 10%-25%

Solar panels energy efficiency is the percentage of sunlight that this particular system converts to electricity. New types of solar panel systems are being developed and improved constantly, and in laboratory conditions engineers are able to reach an efficiency of 42%. While it's an impressive number, in general, the efficiency of most solar panels varies around 10%-25%. The performance of solar panels depends on a set of factors, such as the type of panels you're using, weather conditions and placement.

It's important to note that the efficiency of a solar panel system is determined in Standard Test Conditions (STC): 77 °F (25 °C) and irradiance of 1,000 W/m2. A solar panel system with efficiency of 20%, for example, gives out 200 Watts from 1 m2. The more efficient a particular system is, the less panels and therefore less space for them you need. It doesn't necessarily mean that you should always go for the most efficient choice: performance of a system greatly affects its cost so in some cases it is smarter to invest in a less efficient model that makes use of empty space on your roof.

In practice, an average annual electricity consumption for a utility customer is nearly 11,000 kWh, according to the US Energy Information Administration, which translates to around 30 kWh daily. 14-26 solar panels can cover these needs. Of course, some panels perform better than others, and their efficiency can also be affected by weather or positioning. Let's look in detail at different factors and their effects. There are several of them:

  • weather conditions and shade
  • positioning and angle
  • type of panels
14-26 solar panels are enough to provide electricity for the needs of an average American homeowner

Bad weather can decrease performance by 10-25%

Let's start with the weather. It's natural to assume that solar panels need sunlight in order to function, but it's more complicated than that. Clouds decrease performance of solar panels by about 10-25% but they don't block solar radiation and invisible light completely. Obviously, at night solar panels don't work, though the moonlight might be able to charge them just a little. Surprisingly, solar energy production drops when it's too hot (generally, efficiency decreases by 1% per degree after 77 Fahrenheit), but some models are designed for especially hot conditions so their efficiency is not affected as much.

Shade is much more problematic for solar panels than the bad weather. If some panels are constantly overshadowed by a tree or another building — for example, 10% of the whole system — it can affect the whole array and decrease the lifespan of the panels. Fallen leaves, dust and dirt are generally not a problem — if panels are tilted, then rain simply washes all away. A thin layer of snow doesn't affect performance as much, but when it gets thicker, efficiency decreases. In cases when your panels are placed on the ground, you generally have to clean them yourself.

South side and 37° angle give the best efficiency in US

The efficiency of your home solar panels is greatly affected by their placement. The preferable side of the roof depends on the hemisphere you're in: south is best for the northern hemisphere and vice versa. In the end not all roofs in the USA make it possible to install solar systems on the south side. You still can choose east or west in that case, but keep in mind that output is decreased by 15% on average. There are, however, some cases where you would prefer east or west over south in the USA — read about it in our article on the direction of solar panels.

Angle is a complicated issue. In STC panels are tilted at 37° — that number represents the latitude of continental US. To put it simply, you can tilt your panels at the same degree as the latitude you're in towards the sun and that should give you the best performance.

Monocrystalline panels are efficient, polycrystalline are cheap

The two most prominent types of solar panels are monocrystalline and polycrystalline. However, recently some new models have emerged and they also have some advantages and disadvantages. Let's look at them in details:

  • Monocrystalline panels are made almost purely of single-crystal silicone. The cells of mono-panels often have rounded edges and are black. The efficiency of monocrystalline cells reaches up to 27% in lab conditions. They perform better in low-light situations (clouds, fall), but making them is a slow and expensive process so they cost a lot. They often become a choice in cases when space is limited.

  • Polycrystalline (or multi crystalline) panels are also made out of silicone though not one but a block of crystals. Poly-cells have a square cut and oftentimes are blue. Their efficiency is lower (around 15-22%), but they cost 20-25% less than monocrystalline panels.

  • Thin films solar panels are more of newcomers to the solar panels market. The idea behind them is that photovoltaic material is applied onto a substrate of metal or glass. These panels are lighter and more flexible, but lifespan is shorter (around 20 years). Their efficiency is comparable to polycrystalline panels

There are also building integrated solar panels that become a separate category. This is the case when photovoltaic panels are built into a “smart” building without affecting its appearance. However, the efficiency of these panels is comparatively lower and the cost to integrate a panel into a wall or a roof is high.

The other thing to keep in mind is reflectance efficiency which is determined by how much sunlight is reflected by protective glass. The less — the better. Today almost all manufacturers use anti-reflective coatings to increase the efficiency of their models.

Efficiency also depends on the technologies that manufacturers use. For example, half-cut cell design, PERC cells, multibusbars increase the conversion rate of a panel. Bifacial panels are a special case. The rear side of a module contributes to overall production and efficiency of these models can go over 25%. However, you have to set up the bifacial modules correctly to maximize the reflectivity of a surface beneath the panels and improve their bifacial gain this way.

Leaders in efficiency on solar market

Most of the time the most efficient solar panel systems for home have the highest power output. Typically, models have a power output with a range from 250 W to 400 W in STC. Here's a few words about brands with the most efficient solar panels on the market:

  • Panasonic, the Japanese company, makes panels that are famous for high power density, which on average is 27% higher compared to the product of other companies. With these panels it is possible to generate about 19% more energy from 1 m2 (11ft2).

  • LG Solar panels are famous for their efficiency that oftentimes surpasses 20%. They've been delivering top-tier quality products for a long time and the warranty for them often goes up to 25 years.

  • Solaria makes their panels both in Korea and the United States and is again famous for efficiency of its product — about 20% above other common panels. Clients also often highlight the design of their solar panels.

These three brands are often in the Top-3 of every solar panels' efficiency chart. However, there are lots of companies that challenge leaders and set new benchmarks. For example, Canadian Solar focused on minimizing the loss of energy during low light conditions (for example, in cloudy days) and as a result some of their panels lose only about 3% of their efficiency during bad weather. Using a search filter on our site, you can take a look at the most powerful panels and you'll often see the name Trina Solar there. This Chinese company is comparatively young (founded in 1997) but their products have already broken the efficiency world records in laboratory testing several times.

In 2020 only 6% of American homeowners already have installed solar panels, according to Pew Research Center's data. But nearly a half of them consider installing a solar panel system in future. For now nothing can stop the rise of solar energy, which means that there are many innovations and breakthroughs yet to come in this field. Engineers work daily on making solar panels more powerful, flexible and cheaper and our mission in this is to help our customers to better understand the ins and outs of solar energy and to provide them with quality products. Consider purchasing a solar panel system yourself? Check out our article "How to size a solar system for your home".

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Illustrations – Marina Fionova

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