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Too much sun: What is temperature coefficient of solar panels

You would assume that solar panels want as much sun as possible — hence their name. In reality, more sunlight means higher temperatures and more energy losses. How high can they get? In this article, we’ll discuss heat, solar panels, temperature coefficient and why it’s important.

Panels are tested at 77 °F cell temperature

The power output of a solar panel in the datasheet is what the panel shows at Standard Test Conditions or STC. STC include irradiance at 1000 W/m² and 45° angle, and 25 °C or 77 °F solar cell temperature.

In the datasheet, you can also find the temperature coefficient of a solar panel. It represents the drop in its production when the module temperature exceeds 25 °C or 77° F. Usually, the coefficient varies between −0.3%/°C and −0.5%/°C.

For each degree after 25 °C, a solar panel loses a fraction of its output. It would seem straightforward if the temperature of the panels was equal to the one of the air. But it never is.

Solar panels get hotter than air

10% — average power losses of a solar panel on a hot summer day

In summer the solar panel temperature can reach 140−150 °F even when it’s only 85 °F outside. At this point the power losses caused by high temperatures become significant.

A lot depends on the installation type. Ground and pole-mounted panels heat up the least — they get only 20−25 °C or 35−45 °F hotter than the air.

Rooftop installations tend to get even hotter. If there is a 5−6 inch gap between panels and the roof, the "bonus" to ambient temperature is at about 30 °C or 55 °F on average. If there is no gap between the roof and the modules, they can get 40 °C or 70 °F hotter than the air. Besides, some roofs heat more than others, depending on the roofing material.

NMOT number shows how hot panels get

Below the temperature coefficient in the datasheet, you can also find NMOT — Nominal Module Operating Temperature. This number represents the temperature reached by cells in a module under 800 W/m² irradiance, air temperature of 20 °C or 68 °F, 1 m/s wind and open backside mounting. Generally, an NMOT or NOCT — Nominal Operating Cell Temperature — varies around 45 °C or 113 °F.

The lower NMOT the better. There are situations when even though the temperature coefficient of a panel is high, it loses less power on hot days because of low NMOT.

Comparing solar panels performance in Phoenix, AZ

Imagine we are in Phoenix, Arizona. It’s June and it’s 86 °F or 30 °C outside. We have a few panels that are installed differently and we want to learn how much power they will lose due to high temperature.

Two panels, same mounting

First, we’ll compare the performance of two different panels that are mounted on a pole and their backsides are exposed to the wind.

The Canadian Solar HiKu panel has -0.34%/°C temperature coefficient and 41 °C (105 °F) NMOT. It is likely to reach around 51 °C or 124 °F cell temperature. The power losses will amount to

(25 °C — 51 °C) × -0.34%/°C = 8.84%

This means that a 400 W panel would lose about 35 W of output because of high temperatures.

The VSUN panel has a -0.32%/°C temperature coefficient and 45 °C (113 °F) NMOT. Let’s say that it will reach 55 °C or 131 °F. Here are the power losses:

(25 °C — 55 °C) × -0.32%/°C = 9.6%

This 400 W panel would lose 40 W of output. This applies not just to a single panel but to the whole system.

One panel, different mounting

What if we have the same panel that is installed differently? Let’s compare the performance of a ground-mounted VSUN panel to two roof-mounted modules.

First goes the ground-mounted VSUN panel. We already know how this one goes — the panel is going to lose 9-10% of its output.

Second, we’ll have a VSUN panel that is mounted on a roof with a gap between the module and the roof. We expect it to reach at least 60 °C or 140 °F. The losses are

25 °C — 60 °C × -0.32%/°C = 11.2%

Third, we’ll have a panel on a roof and there is no gap in between. The panel should reach 70 °C or 158 °F. The losses are

25 °C — 70 °C × -0.32%/°C = 14.4%

These calculations are, of course, approximate. The irradiance levels change throughout the day, as do the temperature and wind. Roofing material can also have an impact on how hot your system gets.

Consider HJT panels for a hot area

High temperatures can bring down the performance of your solar system by 5-15%. Moreover, panels deteriorate faster in hot areas. How can you minimize power losses in such conditions? Here is what you can do when planning your system:
• Look for panels with heterojunction cells. Panasonic and REC are the brands that use HJT technology. It brings the temperature coefficient down to -0.25%/°C. It’s as good as it gets.
• All-black modules look better on the roof but they tend to absorb more heat. A panel with a lighter-colored backsheet and a silver frame performs better at high temperatures.
• If you plan to put panels on your roof, think about the racking system that leaves a 4-6 inch gap between panels and the roof. Ground-mounted systems suffer less from heat losses.

Illustrations – Natalya Absalyamova

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