It’s common for solar panel buyers to wonder about all the confusing words and terms on their panels’ spec sheets. Words like temperature coefficient, wattage, and power tolerance can be troublesome to understand for average solar panel buyers. What do these terms mean? Which of these ratings is the most important?

To understand how solar panels work, you must have a grasp of solar panel ratings. These ratings can help you make an informed decision if you are thinking about buying solar panels. You’ll be able to evaluate different solar panels and settle for the one that suits your needs.

Standard Test Conditions (STC)

Sun on Solar PanelsAll solar panels have published power ratings. This power is rated under STC. By adding up the rated power of individual solar panels, you’ll have a solar panel system’s peak rating.

Solar panel manufacturers use this rating to evaluate panels during a process called flash testing. This process is conducted in a solar simulator known as a flash tester. Here, solar panels are exposed to 1000 watts per square meter of artificial sunlight. The atmospheric density and temperature are set at 1.5 and 77 °F, respectively.

The output from these tests under standard test conditions gives the solar panels accurate ratings. All solar panels are rated using the same criteria. This implies that 100 watts from one solar maker will produce 100 watts from a different solar panel manufacturer under standard test conditions.

Nominal Operating Cell Temperature (NOCT)

STC barely resembles the real world. For this reason, municipalities and utilities have come up with Nominal Operating Cell Temperature (NOCT) that resembles the actual conditions of the real world. This rating helps these agencies to issue tax credits and rebates more accurately.

The conditions under NOCT are:

  • Sunlight irradiance of 800 watts per square meter
  • Wind velocity of 2.24 mph with the panel’s rear side open to the wind
  • Air temperature of around 68°F

Given that solar cells are usually black or blue, they can heat up during sunny days. Here, the average cell temperature is approximately 118.4 °F. This isn’t the temperature of air but the solar cells.

Remember, all solar panels typically lose voltage under heat. For this reason, it is advisable to keep the back of your solar panel open for the wind. NOCT ratings mimic real-world conditions.


The wattage represents the expected power output of a solar panel. Solar panel watts are measured under ideal temperature and sunlight conditions, with most solar panels rated between 250 and 400 watts. Still, there are solar panels with higher wattage.

Higher watt modules translate to higher efficiency ratings. They require fewer modules to meet your energy needs. Generally, the wattage of your solar panel system primarily depends on the price of your system.

To calculate wattage, multiply the amps and total volts of the solar panels. Volts refer to the electrical force produced by solar panels, while amps represent overall energy consumption.

Efficiency Ratings

This rating describes the ability of a solar panel to convert the sun’s energy into usable electricity. It measures the irradiation that falls on a solar panel’s surface. For electricity to be generated, sunlight has to hit the surface of the solar panel.

Different solar panels may have different efficiency ratings. A solar panel with a higher efficiency rating will generate more electricity than one with a lower rating under the same amount of sunlight.

This rating is determined by how the panel’s photovoltaic cells produce electricity. The efficiency ratings for most panels range between 15% and 20%.

Temperature Coefficient

Solar PanelEver wondered how well your solar panels will perform in less ideal conditions? Temperature coefficients tell you what to expect in less favorable circumstances.

Just like any other electrical appliance, solar panels operate optimally when kept in cool conditions. They work efficiently at around 77° F.

This coefficient alerts you on what to expect during hot summer days. It gives you an idea of the panel’s performance degradation. For each degree above 77° F, expect electricity production to drop by the coefficient’s value. If your solar panel has a lower temperature coefficient, the better.


During the production of solar panels, some inevitable variations that affect energy output are introduced. This rating indicates the variation between a solar panel’s nameplate rating and its output in standard test conditions.

Power tolerance doesn’t significantly affect power production. It is negligible—still, the smaller your solar panels’ variation, the better.

For instance, a solar panel with a 250 watts rating and a +/=5% tolerance may generate power ranging from 237.5 to 262.5 watts.

Solar Panel Tiers

When researching which solar panels to buy, you’ll hear manufacturers say tier 1, tier 2, and tier 3. These terms generally classify solar panel manufacturers.

Tier 1

Solar EnergyThis solar panel manufacturer class has reputable brands with a good reputation for the solar panels they produce. All of the solar panel manufacturers in this category have been in production for over five years. Solar panels from these companies are low risk.

Some of these companies are publicly listed on stock exchanges. They employ advanced robotic processes when producing solar panels. Here, manufacturing of all solar panel components, including wafers, is done in-house.

Tier 1 companies invest a lot of money in research and development and branding. They are product innovators.

Products from tier 1 companies are pretty more expensive than those from tier 2 and tier 3.

Tier 2

These are smaller players in the solar panel business who have been in business for less than five years. Tier 2 manufacturers put little to no investment in research and development.

They rely on manual labor to get most work done, translating to a higher risk of fault. These manufacturers only use partial robotics in the production of solar panels. Tier 2 manufacturers most often buy components from tier 1 manufacturers.

Tier 3

Tier 3 manufacturers are young companies. These are manufacturers who’ve been producing solar panels for 1-2 years. Humans mainly handle their manufacturing processes.

Tier 3 manufacturers don’t invest in any research and development programs. They only assemble panels they acquire from tier 1 manufacturers. These companies have no capacity to manufacture silicon cells.

Sun Hours

Solar panels don’t receive sunlight throughout the day. They only receive the sun’s energy when they are facing the sun. This is when solar panels receive the most sunlight. Typically, this happens at midday.

A peak sun hour is a rating describing the amount of sunlight (solar irradiance) in a specific area. It is the intensity of sunlight that reaches approximately 1000 watts of energy per square meter. This is around 10.5 feet.

As you get closer to the equator, the number of peak sun hours increases. Your solar panel may get close to 7 hours of sunlight daily. Nonetheless, the average number of peak sun hours in most US areas is around 3-5 hours.


Every solar panel buyer wants to buy the best solar panels system for their home or business. Regrettably, the terms used in the solar industry are far beyond the understanding of most solar buyers. This makes it even harder for buyers to make informed decisions about the products they want to settle on.

Many solar buyers only know about efficiency ratings. Different ratings can help you understand what you need. It’s essential to be aware of other ratings that will affect your solar panels, including wattage and temperature coefficient.

Finding out about these ratings gives you a better hand when selecting your solar panels. You’ll be able to choose superior solar panels that will translate to a high return on investment.