Which solar panels work best in cloudy conditions?

Sunny days are great for solar, of course, but the sun doesn't always shine. Are there solar panels that are specifically designed for cloudy weather?

Photo of clouds.

Solar homeowners always want to see their solar panels generating as much electricity as possible. But even if you live in one of the sunniest cities in the country, there will always be cloudy days.

Or maybe you live in a place where the weather is more temperate, and you experience a mix of sun and cloud. If you want to go solar, you might wonder if certain solar panels perform better than others in cloudy conditions.

The answer is a bit complicated. While there is a testing standard for low irradiance conditions that replicate a cloudy day, few panel manufacturers test and report their panel performance under this standard. This means that you will probably resort to simply looking for the highest efficiency solar panels that fit your budget.

There are a number of newer technologies that are being used to squeeze more efficiency out of solar panels. This article will talk about these, but first let’s cover some basics about solar panel specifications, and how to read a solar panel datasheet.

How do I read a solar panel datasheet?

Looking over solar panel specifications, with its listing of volts, watts, and other cryptic figures might be a little intimidating if you don’t have an electrical background, but I promise that you don’t need to be an electrical engineer to make sense of it all. There’s just a few things you need to know and look out for.

Watts are a measurement of electrical power. The wattage of a solar panel indicates how much electricity it can generate under ideal conditions.

STC, or Standard Test Conditions, is a test that shows how much power a solar panel generates with a solar cell temperature of 25°C and a light intensity of 1,000 W/m² (watts per square meter). These are ideal conditions that you might experience only on a cold, sunny day. The STC rating of a solar panels is its nameplate rating (ie. what’s printed on the box). So if a solar panel is advertised as a 350 Watt panel, that is the STC rating.

NOTC, or Nominal Operating Cell Temperature, tests a solar panel under more realistic conditions. The test setup is 800 W/m² irradiance and 20°C ambient air temperature. The NOTC output of a panel (in Watts) will always be lower than the STC rating - usually in the ballpark of 20% lower.

All these numbers are things you’ll find on a solar panel’s product sheet, and knowing these are enough to get you through this article. To learn a little more about this topic, you can read our article on understanding solar panel specifications.

What is a low irradiance condition?

Low irradiance means dim sunlight. You might think that if you cut the intensity of incoming sunlight by half, the power output of a solar panel would also drop by half. That is usually true, but as the light drops to even lower levels, power output starts to drop off non-linearly.

That is, if your sunlight is only 20% as bright, the power output of a solar panel might drop to less than 20%.

A low irradiance test shows you how a solar panel behaves under typically cloudy conditions by taking the standard NOTC test and dropping the light intensity from 800 W/m² to 200 W/m². (Cutting the incoming light to one-fourth has approximately the same effect as a fully overcast day.) A solar panel that performs poorly in cloudy conditions will show non-linear power loss under these dim light conditions.

SunPower panels and low irradiance test data

Unfortunately, the only manufacturer I’m aware of that publishes low irradiance test data is SunPower, one of the most popular premium panel brands. The datasheets for their panels include two tables: performance of the panels at the NOCT standard of 800 W/m², and another at 200 W/m².

For example, here’s a section of the datasheet for a SunPower X-Series panel (highlighting mine):

SunPower X-Series

In the case of the first panel listed, you can see that the nameplate rating is 470 watts, but the NOCT rating is 356 watts, reflecting the more realistic performance rating that the NOCT test gives.

The datasheet then shows how well the panel performs with just 1/4 of the light:

SunPower X-Series

As you can see, the power drops as expected, and in a roughly linear fashion. That is, that same panel generates 90.6 watts at 200 W/m² - approximately 1/4th of what it generated a 800 W/m².

This tells you that you can expect this panel to still maintain high efficiency on cloudy day. To explore SunPower’s specs on your own, you need to check out their supplemental tech specs section. This data isn’t listed with their regular spec sheets.

Unfortunately, I haven’t been able to find another manufacturer that publishes detailed low irradiance data. If you do, let me know.

Technologies that improve solar panel performance on cloudy days

Given the lack of concrete data on how different solar panels actually perform in cloudy conditions, how do consumers choose?

While some manufacturers claim that certain technologies perform better under low light conditions - such as some types of thin-film solar - data shows that to not be the case.

Instead, consumers who live in less sunny conditions and want to maximize their electricity production while using the minimum roof space should simply focus on choosing the highest efficiency panels they can.

Typically, this means choosing panels that use monocrystalline rather than polycrystalline silicon cells (although technology has improved poly cells to the point where they are nearly as powerful as mono cells).

In addition, there are many other technologies that manufacturers use to improve the efficiency of silicon solar cells, such as PERC, bifacial panels, half-cut cells, and heterojunction cells.

None of these technologies specifically turn a regular solar panel into a “cloudy day” solar panel. Rather, they are some of the technologies that are currently popular among manufacturers in the race to produce ever cheaper and better solar panels.

Finding the highest efficiency solar panels for your rooftop

If you want to install solar panels on your home but live in a place where it isn’t sunny all the time, such as the northern parts of the continental United States, often the best way to maximize your electricity generation is to simply add more solar panels. Even though this means more panels and racking equipment, often the lower cost of budget solar panels means that you end up with a lower price-per-watt for your system.

However, not everyone has the luxury of being able to add more panels to their roof. Often, homeowners are working with limited roof space and want to be able to fit as many solar panels as they can into limited space.

If that’s the case for you, check out our article on the highest efficiency solar panels that are currently available. The best panels panels approach 23% efficiency, and many are available that exceed 20% efficiency.

#Panel Efficiency

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