How Efficient Is A Solar Panel On A Cloudy Day?

Solar panels convert sunlight into electricity using photovoltaic (PV) cells made of semiconducting materials like silicon. When sunlight hits the PV cells, electrons are knocked loose and flow as direct current, which is then converted into alternating current using inverters for powering homes and buildings. The efficiency of a solar panel determines how much of the sunlight striking the panel is converted into usable electricity.

Clouds and overcast weather reduce the amount of direct sunlight reaching solar panels, which directly impacts their efficiency. On very cloudy days, the solar irradiance (a measure of sunlight intensity) can decrease by 75% or more compared to clear sunny days. This significantly lowers the amount of electricity that can be generated by the solar panels. The exact efficiency loss depends on the cloud cover density and thickness. Thinner high clouds allow more light through than thick storm clouds. Understanding how much efficiency is lost on cloudy days is important for properly sizing solar panel systems.

How Solar Panels Work

Solar panels are made up of solar photovoltaic cells (solar cells) that convert sunlight into direct current (DC) electricity through the photovoltaic effect. When sunlight hits the solar cells, it excites the electrons and causes them to flow, generating electricity (Source). The more intense the sunlight, the greater the electricity produced.

Solar cells are made from semiconductor materials like silicon that are treated to form an electric field. The most common material for solar cells is crystalline silicon. Multiple solar cells are connected together and encapsulated to form a solar photovoltaic module, or solar panel (Source).

The DC current generated by solar panels must be converted into alternating current (AC) using an inverter for practical use in homes and on the grid. Solar inverters transform the DC output into AC and also convert the voltage to the proper level for backfeeding into a home electrical panel or the utility grid.

Measuring Solar Panel Efficiency

The efficiency of a solar panel indicates what percentage of sunlight can be converted into electricity. It is measured as a ratio of the electrical power output divided by the solar power input at standard test conditions. Most residential solar panels on the market today have efficiencies ranging from 15% to 22%, with an average around 18% efficiency (source). Higher efficiency solar panels are able to convert a greater proportion of sunlight into electricity. For example, a 300 watt solar panel with 20% efficiency will produce 60 watts per square meter, while a 16% efficient 300 watt panel would only produce 48 watts per square meter from the same amount of sunlight.

Therefore, higher efficiency solar panels take up less space and can generate more electricity over their lifetime. However, higher efficiency panels are usually more expensive. So system designers need to balance efficiency, space constraints, and budget when selecting panels.

Why Clouds Reduce Efficiency

Clouds significantly reduce the efficiency of solar panels primarily because they block and scatter sunlight before it reaches the panels.

When sunlight passes through clouds, the water droplets and ice crystals scatter the light, reducing its intensity. This diffuse, scattered light becomes harder for solar cells to convert into electricity compared to direct sunlight.

In addition, clouds physically block a portion of the sun’s rays from reaching the solar panels at all. Thicker clouds can block a larger percentage of sunlight.

According to a study by the National Renewable Energy Laboratory (NREL), morning and evening clouds have a greater impact on efficiency than midday clouds. This is because when the sun is low on the horizon, its rays have to pass through more atmosphere and are more easily blocked by clouds (source).

Overall, clouds reduce the intensity and directness of sunlight reaching solar panels, decreasing how much electricity they can produce. Thicker and more dispersed cloud coverage causes greater reductions in efficiency.

Estimating Efficiency Loss

Solar panels experience reduced efficiency on cloudy days compared to full sunshine conditions. Research shows that on average, efficiency drops 10-25% during cloudy weather (source). The exact efficiency loss depends on the cloud cover density. Light, scattered clouds have less of an impact on efficiency than heavy overcast skies.

One study in Iraq examined how dust accumulation impacts solar panel output. They found over a two month period, solar panel output dropped 15% on clear days and 25% on cloudy days compared to freshly cleaned panels (source). This demonstrates the compounding effect clouds have on reducing solar efficiency. While dust buildup causes steady efficiency decline, clouds can lead to more acute, daily drops in power generation.

Overall, estimating exact efficiency reductions from clouds is challenging due to variations in cloud cover density and composition. However, the research indicates solar panels operate at 10-25% below their rated output capabilities on overcast days. Lighter cloud cover has a more modest impact, while very dense clouds can cut power production by up to 25%.

Other Factors Impacting Efficiency

In addition to clouds, there are other environmental factors that can affect the efficiency of solar panels. Two notable ones are air pollution/dust and ambient temperature.

Air pollution and dust that accumulates on solar panels can block some of the sunlight from reaching the panels, reducing power output. This is especially an issue in urban areas and industrial zones where pollution levels tend to be higher. Studies have shown drops in efficiency of up to 50% due to heavy soiling on panels [1]. Regular cleaning and maintenance is therefore essential.

Ambient temperature also impacts how much electricity solar panels can produce. Solar cells work best at cooler temperatures. On very hot days, efficiency can drop by 10-25% [2]. Proper solar panel ventilation and cooling is important, especially in hot climates.

Improving Low Light Performance

There are a few ways to improve the efficiency of solar panels in low light conditions like cloudy days. One option is to use mono-crystalline solar cells rather than poly-crystalline cells. According to Panasonic, mono-crystalline cells work better in low light because their chemical structure is more efficient at converting diffuse light into electricity.

Another way to boost efficiency on cloudy days is by using solar trackers. Solar trackers move the panels to follow the sun throughout the day, helping maximize the light exposure even when the sunlight is partially blocked by clouds. As noted in this Solar Alliance article, trackers can increase the energy output of solar panels by up to 40% on cloudy days compared to fixed mount systems.

Geographic Differences

Solar panel efficiency can vary significantly based on geographic location. Cloudier climates in particular see greater reductions in solar panel efficiency on overcast days. Areas like the Pacific Northwest that have more rainy and cloudy weather overall will generate less solar energy from panels than sunnier climates like the desert southwest (source).

In addition to average cloud cover, seasonal weather patterns impact solar output throughout the year. In northern latitudes, solar panels are less efficient during the winter when there are fewer daylight hours and more overcast skies. The summer months with longer days and clearer skies allow for greater solar production. Southern regions with more temperate climates see less extreme seasonal swings in solar panel efficiency (source). Overall, geographic location and regional weather patterns play a major role in determining real-world solar panel efficiency.

Energy Storage and Net Metering

Batteries can store excess solar power generated on sunny days for use during cloudy periods when solar panel output is reduced. Home solar battery systems like the Tesla Powerwall allow solar homeowners to store solar energy in a battery for later use (SolarReviews). This provides a backup power source on cloudy days when the solar panels are not generating as much electricity.

Net metering is another option for dealing with variable solar panel output. With net metering, any excess solar electricity generated is fed back into the grid and the homeowner receives a credit from their utility company. This credit can then be used when the home is drawing more power than the solar panels are generating, for example on cloudy days (Reddit). Net metering effectively allows the grid to act as a battery for stored solar energy.

Battery storage and net metering both help solar panel systems provide power during cloudy weather. Batteries provide backup power onsite, while net metering uses the grid as supplemental storage. Homeowners can choose between these options depending on their specific needs and local net metering policies (Canary Media).


In summary, solar panels can still generate electricity on cloudy days, but at a reduced efficiency compared to sunny days. The amount of efficiency loss depends on the cloud cover thickness and type of clouds, with thicker storm clouds blocking more sunlight than thin wispy clouds. On average, solar panels may operate at 10-25% reduced efficiency on a fully cloudy day. However, innovative solar cell materials and panel designs are improving low-light performance. Even with some efficiency loss, solar power remains a clean, renewable energy source worth considering in many regions. The viability depends on full sun exposure, climate, and local electricity rates. With energy storage like batteries, solar power can still provide clean energy during cloudy spells.

Solar energy remains an important part of our sustainable future, even on cloudy days. While output is reduced, solar panels can still generate ample electricity from diffuse light coming through clouds. Plus, solar power systems work at full capacity during sunny weather, offsetting losses at other times. With the right expectations and system design, residential and commercial solar installations are still worthwhile investments in many areas despite some inevitable efficiency loss on cloudy days.

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