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How are solar panels rated?

4 min read

Rating systems can vary a lot from one industry or niche to the next. We all understand intuitively the 5-star rating system that online shopping retailers and review sites use.

We’re also all familiar with the school-like grade ratings that movie review sites attribute to both professional critics and movie-goers alike. Maybe the critics give a certain movie a C, but most people who went to see the movie for fun say it was more of a B+.

Or, there are the classic “thumbs up” or “thumbs down” reviews, making your decision about which flick to spend a couple of hours of your Saturday night on easier. What about solar panels? How are solar panels rated?

How are solar panels rated?

How can you know that you’re getting the best solar panels for your home?

In this article, we’ll talk about the process that solar panel manufacturers go through and the standards the industry has established to keep solar panel ratings consistent.

Standard Test Conditions (STC)

Standard Test Conditions (STC) are the industry-standard lab testing conditions that solar panels go through. Having set criteria for all solar panels helps to ensure that all panels are held to the same guidelines and, essentially, that the same rating system is being used across the board.

These conditions occur in a laboratory setting where technicians can control and monitor the environment to certain specifications. This keeps things consistent. Each solar panel is tested and rated according to how much power output it produces under the STC.

What does the STC consist of?

The temperature of the solar cells

By measuring the solar panel’s electricity when the solar cells within the panel are at a steady 77 degrees Fahrenheit, technicians can get a good read for when the panels are operating at an ideal temperature.

It's important to note that this temperature is measured at the solar cells themselves, not at the ambient air surrounding the panel.

How much light energy solar panels receive

The industry term here is solar irradiance (SI). What this means is exactly how much solar radiation, or light, is falling on a specific area (a solar panel) at a particular time.

1,000 watts per square meter is an ideal solar irradiance level. So, this is the wavelength used in standard test conditions.

The angle of the sun hitting solar panels

Another important standard of measurement is known as the Mass of the Air, or Air Mass (AM). It’s a somewhat confusing term, used to describe the amount of light that passes through the atmosphere before reaching the solar panel on the surface of the Earth.

What it really comes down to is the angle of the sun in relation to the solar panel here on Earth. When the sun is directly overhead, it hits the solar panel most directly and at the optimal angle ratio of solar radiation and solar panel. This is AM1.5.

PV USA Test Conditions - (PTC)

Just slightly different than the STC are the somewhat more realistic conditions used in PV USA Test Conditions (PTC).

These conditions require some of the same things as the STC, including the same measurements for solar irradiance (1,000 watts per square meter) and the angle of the sun hitting the panels (AM1.5).

Where the two differ is in the temperature. While STC guidelines set solar cell temperature to 77 degrees Fahrenheit, PTC guidelines set ambient air temperature (the temperature surrounding the solar panels) at 68 degrees Fahrenheit.

The solar cell temperature itself is set to 113 degrees Fahrenheit. In addition to this, wind speed is set to 2.2 miles per hour. This is done to mimic the real-world conditions solar panels are likely to live in under the hot sun and with a cooling breeze.

You can ask for both the STC and PTC ratings for whichever solar panels you're interested in and compare the two. Of course, there are some other things that come into play in solar panel production as well.

Other variables in solar panel performance

Solar panel efficiency

This is the number indicating the percentage of solar energy that a particular solar panel can convert into usable electricity. Efficiency ratings can generally vary from 15 to 18 percent.

Degradation rate

Over time, like anything in life and with electronic equipment in particular, solar panels will degrade in the amount of power they’re able to produce. A degradation rate will give you a good indication of the projected lifespan of your solar panels.

For more information on factors that can affect your solar panel production, read this post on how to calculate solar panel output.

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