Solar Panel Efficiency — Understanding STC and PTC Ratings

Are you comparing solar panel options? Efficiency — the panels’ ability to convert sunlight into energy — is a feature you should consider.

Before putting photovoltaic panels on the consumer marketplace, manufacturers test them and assign power ratings that indicate panel energy output (in watts). As you look at solar panel efficiency specifications, you’ll see both STC and PTC ratings.

solar panel efficiency

But what are these measurements, and what do they really mean in terms of a real-world PV installation?

What Are Solar Panel STC Ratings?

If you want an easy way to compare the efficiency of one solar panel to another, look for the STC rating.

Standard Test Conditions (STC) refers to the fixed set of laboratory conditions under which every solar module is tested. Manufacturers use STC testing to ensure that photovoltaic panels with similar energy output can be sold and used together.

The STC rating of a solar module is determined by carefully controlling light and temperature in the testing environment.

The light source in the laboratory is calibrated so that precisely 1,000 watts per square meter of solar light falls on the photovoltaic panel. The temperature of the solar cells and the ambient room temperature are both set at 77 degrees.

STC ratings are always higher than PTC, because they are based on the modules’ instant output under ideal conditions.

What Are Solar Panel PTC Ratings?

If you want to know how photovoltaic panels will perform in the real world, look for their PTC ratings.

In the 1990s, the National Renewable Energy Laboratory (NREL) developed a set of PV test parameters designed to mimic real-life atmospheric conditions. The Photovoltaics for Utility Scale Application Test Conditions, or PTC, is significantly different than the STC testing protocols.

Under PTC, lighting conditions are the same as the STC, but the solar module is heated to a more realistic operating temperature of 113 degrees. In addition, the ambient temperature in the laboratory is set at 68 degrees. Finally, since photovoltaic modules in the real world are exposed to wind, PTC testing keeps the air moving at 2.2 mph.

Generally, if no PTC value is listed in the specifications for a photovoltaic panel, you can expect it to be about 10 to 15 percent less efficient than the STC rating.

Factors that Affect Solar Panel Efficiency

Although PTC ratings offer a realistic view of photovoltaic output, they don’t indicate exact energy production. Several specific factors — known as derate factors — can reduce the amount of solar electricity created by a PV system.

Some factors, like shading on the photovoltaic panels, can be eliminated through the PV installation process. Other factors, including energy conversion losses, wiring inefficiencies and module heating, are unavoidable. Overall, derate factors typically decrease solar panel efficiency by about 14 percent.

It is important to note that module efficiency ratings reflect system performance in the “perfect world” of the lab environment. The decrease in efficiency between the lab and your roof is a little like the actual miles-per-gallon you get in your car, compared to the window sticker ratings. The primary value of these ratings is for comparing modules and choosing the best options for your system.

An experienced photovoltaic contractor can design and install a PV system that maximizes your energy production, whatever your needs. Intermountain Wind & Solar offers free consultations to homeowners and businesses throughout Idaho, Utah, Wyoming, Colorado and Nevada. Contact us today to learn more about solar panel efficiency.