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All solar panels need is the sun — but what if it’s behind the clouds? Is it worth purchasing a solar system if you live in a rainy area? In this article, we’ll talk about how solar panels work on cloudy days and why the number of clear days in an area is not the most important factor when going solar.
As long as there is daylight, panels keep on working. The production drops by 10% to 40% on cloudy days, depending on the properties of your panels and the thickness of your clouds. If the output of your solar system is at 5 kW when it’s sunny, this number is going drop to 3-4 kW on a gloomy day.
So the answer to the question “Do solar panels work on cloudy days” is a “Yes” — but how? Panels absorb not only visible sunlight. Solar radiation in different ranges of wavelengths still penetrates clouds and lands on our PV modules, even if we don’t see it.
A part of the answer is that panels don't necessarily need direct sunlight to function. They produce the most when sunlight strikes them at a 90° angle. This is why positioning and angle are so important for a solar installation. But aside from direct radiation, there is also diffuse and reflected radiation, and panels intake all of it. As long as it is daylight, solar panels continue to produce energy.
What you really don’t want is for your solar panels to be in a shade. One shaded cell of a panel can decrease its production by up to 33%. What’s worse, a string inverter may lower the production of each solar panel in your array down to the level of one shaded module. You can set up multiple strings or add solar optimizers to your system to prevent it from happening.
Solar panels do worse in highly polluted cities. Smog acts as cloud cover and lowers the production of solar systems by 10 to 40%. The losses increase in the fall and winter because a lot of coal is used at that time for heating.
The effect of air pollution on solar production was explored by experts from Princetown University. They studied Chinese solar farms and found out that their production decreased as much as 35% or 1.5 kWh per m² per day. For owners of these farms, a 10% production decrease is within their profit margin. For this reason, solar projects become less attractive to investors in areas with high air pollution.
Do solar panels work in the rain then? The panels don’t care about water, as long as it doesn’t get inside the module. They can even withstand hail: PV modules are certified to survive the direct impact of hailstones about 1 inch in diameter falling at 50 mph.
The mechanical tolerance of a solar panel is listed in the datasheet. It specifies the maximum front load that a panel can bear. For example, 2400 Pa — a common value for panels — is equivalent to 140 mph wind. It takes a major category 4 hurricane to break down your solar system.
Solar panels perform worse when it is too hot. When a cell temperature goes over 25 °C or 77° F, the production starts to drop. Read our article on temperature coefficient to learn more about heat losses of a panel.
average power losses of a solar panel on a hot summer day
The best place for solar panels is somewhere sunny and cold. Still, panels with heterojunction technology or HJT cells, like those offered by REC and Panasonic, are great at dealing with high temperatures. If you plan to get a system in a hot place like Phoenix or Las Vegas (Nevada), these are the go-to choice for you.
Sometimes the production of solar panels on cloudy days is higher than normal. Reddit user achinistcalculator wrote in r/solar: “Why do days with thin clouds like these have better peak output than days with no clouds? 1312w with no clouds on Thursday vs 1574 when the clouds started to roll in on Friday”.
Some suggested that increased production has to do with a lower temperature. Here's what user by the name ceraexx wrote:
“Possibly edge of cloud effect. It magnifies the irradiance. Normally on a sunny summer day, you might get 1000 W/m², but you can get 1400 or so with edge of cloud. Depending on how it hits and for how long it might be higher…"
Cloud edge effect — sudden increase in irradiance due to reflection of the passing cloud focusing more sunlight to the solar array
Ceraexx also added later that time of the year could be a factor: “In winter your daytime irradiance is lower due to the angle of the sun, so if your reference output day of 1312 W was a month ago, your average irradiance may have been 680 W/m², but now it could be 800 W/m²”.
Irradiance levels change throughout the year. Your solar system is going to produce about 50% more energy in June than in December. To make use of use, some solar owners add extra loads in summer, such as an AC unit.
On paper, you would get the most out of your solar panel system where the average number of clear days per year is the highest. Here’s the list of the sunniest states of America, according to the National Climatic Data Center:
- Arizona — 193 clear days a year on average
- New Mexico — 167 clear days a year
- Nevada — 158 clear days a year
- California — 146 clear days a year
- Oklahoma — 139 clear days a year
Not surprisingly, the town leading by total installed solar capacity is Los Angeles, California — it’s easy and self-evident to go solar in a place where the sun is. But when it comes to solar panels it’s not only important how much energy they produce, but how much money you save through them. That depends on electricity prices in different states of America.
Let’s look at the least sunny states of the USA, where a solar panel system at first doesn’t seem like a very good option:
- Washington — 58 clear days a year on average
- Vermont — 58 clear days a year
- West Virginia — 60 clear days a year
- Alaska — 63 clear days a year
- New York — 63 clear days a year
Here is the funny thing. States, where your solar system is going to pay for itself in the least time, are not the sunniest. For example, a solar panel system in New York city gives one of the highest return rates in the USA, although the NY state is the 5th in the list of the least sunny states.
average payback time of a solar system in New York City
The reason for it is the high electricity cost. It turns out that even if you are getting less energy through the solar panel system due to bad weather in your area, you still might be saving more money than those who live in a sunnier state.
Right now the states, where solar energy pays for itself faster than everywhere else, are the following:
- Hawaii — average payback time is around 5 years, but the annual number of clear days is only 90.
- Washington DC — 5-year average payback period, 58 clear days a year.
- Massachusetts — 6-year average payback period, 98 clear days a year
- Rhode Island — 6-year average payback period, 98 clear days a year
- New York — 7-year average payback period, 63 clear days a year
The deciding factors that make the switch to solar energy more or less appealing in one or the other state are electricity prices and programs and incentives. The weather is also important but it shouldn’t be overestimated.
Electricity prices change at different rates in every state and the list of cities where solar energy gives the most benefits might change in the future. The steady rise in electricity prices all over America, however, makes the shift to solar energy even more appealing.
15.96 cents per kWh
average residential electricity rate in the U.S. in 2023
The price of electricity went up by 18% in the last four years, according to the U.S. Energy Information Administration. An average American would pay 13.16 cents per kWh in 2020. In 2023 the number is close to 16 cents per kWh. The higher electricity prices climb, the faster solar panel systems pay for themselves and the more money they bring in to their owners.
To sum it up: solar panels can pay off faster in cloudy states. The value they bring in depends not only on the amount of energy they produce but also on the price of that energy. Given the rising cost of electricity, it’s important to act quickly. Read our article on Federal Solar Tax Credit to find out how you can save more money on buying a solar panel system.
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