Solar panel temperature coefficient explained

On a hot day, not all solar panels perform the same. Here's how to find out how your solar panel will handle the heat.

Illustration of two thermometers

If you’ve ever looked at the product sheet for a solar panel, you may have been confronted by tables of cryptic numbers and electrical terminology.

Unless you’re an electrician or electrical engineer, you’ll probably find these a little bewildering. Fortunately, the average consumer can ignore most of them. Unless you’re an installer or hobbyist, many of the numbers are electrical specs that won’t matter to you.

One exception, however, is power temperature coefficient. This number tells you how much electricity loss you can expect from the panel when the temperature of the solar cells inside get hotter. It’s a useful figure to know because two different panels might have the same power rating, but the one with a better temperature coefficient will tend to generate more electricity in the real world.

It’s a little bit technical, but easy to understand once you understand what it means. We’ll look at some examples, but first let’s understand how temperature affects the performance of a solar panel.

Why does temperature affect the power output of a solar panel?

All electrical conductors experience higher electrical resistance as the temperature increases. This means that they conduct less electricity as they get hotter, and turn more of the electricity into wasted heat. Everything from overhead power lines to computer processors are affected this way.

Conversely, if you cool an electrical conductor down, it becomes more efficient. If you make some conductors cold enough, they become a superconductor in which the electrical resistance drops to zero.

We don’t have superconducting solar panels yet, but the ordinary panels you would put on your roof will perform better when it’s cold. The power temperature coefficient of the panel tells you exactly how well the panel handles the heat.

How to find the power temperature coefficient for your solar panel

To get started, look up the specification sheet for your solar panel. You may have to dig around, but you can usually find this on the manufacturer’s website.

Once you’ve found it, look it over until you find a section with figures for temperature coefficient. Manufacturers may label this in different ways, so look closely. For example, SunPower has it listed as “Basic Temperature Data”. Here’s an example from a Trina Solar datasheet:

Trina Solar temperature ratings
Trina Solar temperature ratings

There are three figures listed related to temperature coefficient:

  • Pmax refers to maximum power output, in Watts
  • Voc is the open circuit voltage, which is the output voltage when there is no load connected to the panel
  • Isc is the short circuit current, which is the number of amps the panel outputs when it is short circuited

Consumers will be interested in Pmax, which is the maximum wattage the panel will produce. The other two figures are of a more technical nature and irrelevant to most homeowners.

Pmax is expressed as a loss in efficiency for every 1 degree Celsius (about 1.8 degrees Fahrenheit) increase in temperature.

So, the table above says that this Trina Solar panel has a power temperature coefficient of -0.36%/°C, which means that it generates 0.36% less electricity for every 1°C that it gets hotter. (The baseline temperature for the measurement is 25°C.)

One thing to know is that this is the temperature of the solar cell and not the air temperature. It could be a cool day, but because a solar cell is dark, it absorbs sunlight and can get a lot hotter than the surrounding air. This means that a panel will experience this impact more on a sunny day than a cloudy one, even if the air temperature is the same.

When does power temperature coefficient matter?

For small-scale installations like rooftop solar, power temperature coefficient is a fairly minor consideration in the overall performance and cost-effectiveness of the system.

Where it starts to really matter is large solar farms, especially in hot climates. At that scale, small differences in performance can have a meaningful impact on profit margins.

However, if you’re a homeowner who is into the technical details of things, this might be something you want to pay attention to, especially if you live in a hot climate such as Southern California or Arizona.

What are some panels that perform well when hot?

You’ll find that solar panels have a wide range of power temperature coefficients. In general, “premium” solar panels with high efficiency also tend to have better performance in the heat. Here’s just a few examples:

PanelPower Temp. Coefficient (loss per 1°C)
REC Alpha-0.26%
SunPower X-Series-0.29%
LG NeON 2-0.34%
SunSpark M2 Poly-0.40%
Phono Solar Mono-0.42%
Kyocera KD 300-80 F Series-0.45%

The REC, SunPower, and LG panels are considered “premium” products while the SunSpark and Kyocera panels are budget-friendly models that I found on an online retailer.

As you can see, the REC Alpha is the best performer in this group, with a much lower loss of performance when it starts to get hot.

Does the thermal performance of a solar panel really matter?

Now that you know what it is, one thing to consider is whether you really care about thermal performance at all.

You might not care. The Phono Solar and Kyocera examples above have relatively poor thermal performance, but they’re budget panels with a wallet-friendly price. If you end up spending less per Watt by going with a budget option, specs like power temperature coefficient might not really matter much.

However, there are a couple situations where you might want to pay attention.

If you’re comparing two panels with a similar wattage and price point, the one with a better power temperature coefficient will likely perform better on your roof.

Another case is if you have limited space on your roof and want to get the maximum power generation from the square footage you have. In this situation, you’ll want to find the most efficient solar panels you can. But as you’ve learned from this article, the headline efficiency numbers that manufacturers advertise don’t give you the whole picture. It’s really a combination of efficiency and power temperature coefficient that will give you a complete picture of a panel’s real world performance.

Fortunately, there’s another set of numbers that manufacturers publish that make all of this a little easier to understand. The advertised wattage of a solar panel is under Standard Test Conditions (STC), but there are other rating systems such as NOTC that better account for thermal performance. This makes it easier to understand how differences in power temperature coefficient translate into wattage.

To learn more about STC, NOTC, and other fun acronyms related to this topic, check out my article on how to read solar panel specifications.

#Specifications #Panel Efficiency

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