Solar panels do work on a cloudy day but at a reduced efficiency. Output may be only 10% to 25% of the solar array rating and exactly how much depends on Cloud Cover Factor (CCF). Paradoxically, solar panel output with passing clouds can be higher than on a sunny day.
The energy from the sun reaches the earth in the form of solar radiation. That means visible light, ultra-violet light, and radio waves among other forms. This is the one fundamental limitation of solar energy.
In this article, we are concerned about how this energy is converted into an electric form using solar panels in different weather conditions such as cloud cover and rain.
I’ll look at how these different weather conditions affect the performance and efficiency of the solar panels.
How much does cloud cover reduce solar power?
It’s evident that on a sunny day, solar panels operate at a higher output than on a cloudy or rainy day, but how much?
When the weather changes, there is a change in how much electric energy is generated by solar panels, simply because all of the sun’s energy can’t reach the panel surface.
Let’s find out how a cloudy day affects solar cell efficiency and output.
Video – Solar Panel Output Comparison Sunny Day vs Cloudy Day
The stock answer of how much power can a cloud-covered solar array generate is between 10% and 25% of it’s optimum rated power output.
I couldn’t find too much hard data about the subject but this research paper explains how the effect of cloud cover on solar panel output can be quantified.
The level of cloud cover is assessed and given a value between 0 and 1. This is called the CCF or Cloud Cover Factor. 0 would a clear sky and 1 would be equivalent to total cloud cover!
The CCF is directly related to the reduction in solar irradiance or the sun’s energy per square meter – the irradiance value is also known as peak-sun-hours.
Solar installers take into account historical cloud cover data when sizing solar systems. This site is an example of cloud cover statistics for continental USA.
Cloud map resources: http://coolwx.com/usstats/cloudstats.php
Do Solar Lights Charge on A Cloudy Day?
This is of concern for those who may want to install solar lights. Alhough the clouds diffuse sunlight so sunlight doesn’t reaching the solar panels directly, some energy still gets through.
Statistically, solar panels can generate about 10-25% of the normal output when it’s cloudy, but is that enough to stop the lights working?
How do solar garden lights work?
The part that gives light is basically an electronic component called a Light Emitting Diode (LED). They use very little power (1.2 volts 50mA) and have a 25 year life.
The LED is power by a 1.2 volt Nickel Metal Hydride (NmH) battery with a capacity of 600 to 2000mAh, depending on the size of the garden light. With a lifetime of at least 1000 cycles, it may need replacing in 3 to 4 years.
There’s a great animation of solar garden lioght operation here.
A single solar cell with specifications of 1.5V, 100mA in full sunshine is enough to recharge the battery and run the LED for up to 15 hours.
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Battery charging circuitry isn’t needed as the solar cell output is too low to ever harm the battery. A simple blocking diode stops the battery from feeding back to the solar cell.
With irradiance average value of 4 hours peak sun hours the solar light would be good for 15 hours but this may not be the case in every location and in all seasons.
Table – Seasonal Irradiance San Francisco, Ca
Seasonal Irradiance for San Francisco by month (Peak-Sun-Hours)
Though the solar lights will charge on a cloudy day, common sense tells us that charging speed will be slower compared to a sunny day for the same amount of exposure time.
For example, if the irradiance falls below a certain level in winter then the battery may only run the led for half the time i.e. 50% of the summer run-time.
Do Solar Panels Work on Rainy Days?
How well do solar panels work on rainy days? How is energy production affected by the rain?
Studies show that the effects of rainy days has a worse effect than cloudy days on solar panel output. This is probably due to the fact that rain comes from clouds, so it adds to sunlight blockage even more.
Solar panels may only generate 10% of their rated power in heavy rain. Yes, they do work in the rain, but not very well at all.
Resource Link: Research into solar panel output in adverse weather conditions.
Solar Panel Efficiency on Cloudy Day
Solar panel efficiency is affected by several factors, such as tilt angle and orientation, but by far the most important thing is irradiance – how much of the sun’s energy reaches the panel surface and is transformed into electricity?
This is why clouds and rain reduce the output so much, the sun’s rays can’t get through.
Irradiance is measure in kWh/m2/day or year, otherwise known as peak-sun-hours, and is generally strongest mid-summer. It reduces in the spring and fall, finally becoming it’s weakest in the winter.
In general, the output level of a solar panel array goes up and down, basically following the irradiance level. However, in certain conditions this is not the case.
Mostly around March a phenomenon called the Edge-of-Cloud Effect can come into effect.
A solar panel array has an STC rating (Standard Test Conditions) in watts which is almost never achieved in practice. Solar system losses reduce power output by about 23%.
As small clouds pass in front of the sun water droplets in the cloud’s leading and trailing edges magnify the sun’s rays. The effect is to boost the solar panel output.
Solar panel temperature is also lower in these months – solar cell efficiency is higher when cooler. It’s not uncommon in these conditions to see power output up around the panel’s STC power ratings.
Do Solar Panels Work in the Snow?
Snow laying on flat solar panels will almost certainly have almost zero output as none of the sun’s enegry can reach the panel surface.
In practice, it’s rare that solar panels are installed flat, unless mounted on an RV or similar roof.
Can solar panels work with snow on them?
A tilt angle is important as this optimizes power output. As solar cells heat up when converting sunshine into energy it’s rare for snow to accumulate – it simply melts and slides off.
White snow all around a solar panel array will cause an albedo effect, where reflected sunlight boosts power output, directing more sunshine onto the panels.
Do Solar Panels Work in the Cold?
Not only do solar panels work in the cold, they actually work better in the cold!
Solar panels are rated according to STC standards, or Standard Test Conditions, which specifies a certain output at a temperature of 25°C. As the temperature rises solar cell conversion efficiency falls.
For every degree above 25°C panel output falls by 0.5%, but does it work the other way? Does efficiency continue to improve as solar panels get colder? Unfortunately not, or we could simply super-cool the cells to generate massive power!
Solar panel power output is maximum at 5°C (41°F). At lower temperatures the efficiency doesn’t continue to improve.
In a nutshell, solar panels work in a variety of weather conditions but at varying rates of efficiency. Solar output on cloudy days will be reduced by an average of 25%, depending on the Cloud Cover Factor (CCF).
A rainy day reduces solar cell output more than just clouds. Rain clouds are usually darker and generation may be down to just 10% of normal output in full sunshine.
Complete snow cover will basically stop all production on a flat panel, but it’s rare to install panels flat. A tilt angle is always recommended for optimum power generation.
That said, professional installers will size the solar system accordingly, taking into account prevailing weather conditions in your locations.
Every location on the planet has a certain average level of irradiance (sun’s energy) measured in kWh/m2/day (or year). This value is also known as peak-sun-hours and is used to size solar panels.