Before you invest in solar, you probably have questions about how much sunlight solar panels need to convert photons into energy.
Solar panels convert particles of light, or photons, into electricity. So, many homeowners wonder what happens at night or when it’s cloudy. The short answer: solar panels don’t produce power at night, but they do work in cloudy weather, just with reduced output. Modern photovoltaic (PV) systems are designed to make the most of available light, and with the right setup (like battery storage), your home can stay powered even after the sun sets.
In this article, we’ll explain the science of solar cells, why partly cloudy days can sometimes outperform clear ones, and how solar battery storage helps power your home after dark.
In this article:
- How solar panels generate electricity
- Solar panel performance at night
- Solar panel performance on cloudy days
- Solar energy options at night or on cloudy days
- Conclusion
- Frequently asked questions
How solar panels generate electricity
To fully understand when solar panels can produce electricity and how much they can generate, it helps to know exactly how they work. Basically, solar panels convert light into electricity — photons, or light particles, dislodge electrons from the silicon panels to start the energy production process. So, while a direct beam of sunlight maximizes the amount of energy your home solar system can produce, the diffuse light of a cloudy day will produce electricity, too.
Here’s what you need to know about how solar panels work, and why it’s important to know the difference between direct and diffuse irradiation.
Photovoltaic effect: the science behind solar panels
Solar panels are made up of a number of solar photovoltaic, or PV, cells that convert sunlight into energy. When photons — aka sunlight — hit the charged semiconductor materials layered into PV cells, the cell’s electrons move, and electrical conductors absorb the electrons, creating a direct current, or DC electricity. This current runs from the cells into the solar panel’s inverter, which turns the DC into the alternating current (AC) electricity that powers your home’s lights and appliances.
Solar panels need photons to work, and in general, the more photons directly hitting the solar cells, the more electricity they produce. And the more solar panels you have collecting those photons, the more energy you can generate. Calculating the solar irradiance, or available solar energy, for your location is one of the first things an installer will do to see if your home is a good fit for solar. Solar irradiance is key to understanding how many solar panels you need to generate enough electricity to power your home.
On bright, sunny days in the summer, when the sun is high in the sky and days are longer, your solar panels will generate the most electricity — even more electricity than your home needs. But your solar panels will work in all kinds of weather conditions.
The tilt angle and spectral response of your solar panels are also factors in solar panel efficiency. Sunlight has a variety of wavelengths, and how PV cells respond to these different wavelengths is called their spectral response. Solar panels are now being designed with layers of cells with different spectral responses to harvest more wavelengths into energy, and optimize their performance. Low-sun angles can also reduce efficiency as the photos travel through more of the panel’s glass, and the earth’s atmosphere, but solar panels can still generate energy when the sun is low on the horizon. They just won’t harvest as much energy.
Direct vs. diffuse light
Total solar irradiance is made up of both direct and diffuse light. Direct irradiance comes from the sunlight shining directly on your solar panels. Diffuse irradiance comes from light that has been scattered by clouds or other particles in the atmosphere.
A mix of clouds and sun can actually improve total solar irradiance, because the clouds can create diffuse irradiance alongside the sun’s direct irradiance. When there’s full cloud cover, solar panels can still generate some energy from diffuse irradiance, but at a reduced rate.
How much is a solar panel’s efficiency reduced under heavy clouds? Here’s an extreme example: Researchers tracking PV efficiency during a hurricane noted that the hurricane’s heavy cloud cover reduced the energy output to 18%-60% of their clear-sky output. (It’s worth noting that rain can be helpful for improving your solar system’s efficiency after the storm, as it will help clean off any dust and debris that might have accumulated on the panels.)
If you’re looking at your panels’ nameplate power rating, you might be wondering how that compares to these real-world conditions. It’s worth noting that even your summertime clear-sky output is likely to be lower than the nameplate power rating, which represents their maximum generating power under ideal conditions. Several factors contribute to this nameplate “power loss,” including temperature, other weather conditions, and more.
Solar panel performance at night
Solar panels need light to operate, but you might still see your system pop on at night. Here’s what you need to know about why your panels won’t produce power you can use at night, and how moonlight can affect your solar system.
Why solar panels don’t produce power at night
Solar cells need sunlight, or photons, to start the process of moving electrons to create a direct current to generate electricity for your home. Without photons, the process can’t start. So at night, your solar panels won’t produce energy.
But there’s a clean-energy solution to this perpetual problem. During the daylight hours, your panels can be used to charge a solar battery that stores enough energy to power your home in the evening after the sun goes down. Read more in our solar energy options section below.
Moonlight and artificial light: negligible output
Once your solar system is installed, you might have full-moon nights when you see a small bump of power coming from your system, or a streetlight might trigger some trivial output. Unfortunately, these weak, diffuse light sources aren’t enough to power your home.
Technically speaking, moonlight is a form of sunlight — it’s reflected from the sun. And an artificial light within the solar panel’s spectral response range could provide enough photons to start the process of creating electricity. But it wouldn’t be much, and it would be an inefficient way to power your system. These light sources are rarely worth harvesting.
Solar panel performance on cloudy days
Solar panels will still generate electricity when it’s cloudy, but they’ll produce less than they do on a sunny day under direct sunlight. Output is generally reduced in silicon PV by 10%-25% on cloudy days, but under heavy overcast skies with no direct irradiance, output could reduce by more than 60%.
The type of solar panels you have can affect your output on cloudy days, as some perform better in low-light conditions. Monocrystalline silicon is more efficient than polycrystalline, but passivated emitter and rear contact, or PERC solar cells, also perform well in low-light conditions. PERC cells have an added layer that reflects unused light back through the cell.
Geographic and seasonal variations
Because regional and seasonal weather changes can have an effect on irradiance, any good solar installer will use tools that help them see how changes to the sun angle in winter, changes to daylight hours, and sunny versus cloudy days in your region will affect your solar production.
The National Solar Radiation Database also helps designers of solar systems understand how regional cloud cover averages will affect the expected output from your solar panels and make adjustments accordingly.
Solar energy options at night or on cloudy days
With rising utility bills and a drive to shift to clean energy sources, many homeowners are now opting for solar with a battery storage system as a smart solution for powering their homes at night. This powerful “solar plus storage” combo is a fast-growing sector of residential solar because it helps reduce dependence on the grid and lowers energy costs during potentially expensive evening hours.
In this section, we’ll cover the basics of solar batteries, net metering, hybrid systems, and solar generators, so you can decide what energy solution makes sense for your home.
Battery storage systems
You can still power your home on solar at night if you have a battery storage system. Home solar systems often produce surplus power during the day that can be used to charge a battery for use at night.
Solar-plus-storage has become a popular green energy option for homeowners for good reason — a 10 kWh battery can power a house that uses 5 kW for about 2 hours, offering good power coverage during the evening — the peak energy use hours for most homes.
Lithium-ion batteries have become more affordable and are a popular choice for homeowners considering battery storage. Other home battery options are available, though, including lead-acid batteries, which are lower cost but have a shorter lifespan, and flow batteries, like zinc-bromide batteries.
Net metering
Net metering lets you export the excess electricity your system generates to the grid during the day to help offset the cost of the electricity you’ll import from the grid at night. Your utility credits you for what you send and charges you for what you use.
However, if your utility uses time-of-use (TOU) rates and/or variable export rates, the math isn’t always equal. You typically get less for the electricity you export during daytime hours than you pay for the electricity you import during expensive evening peaks. That’s why net metering alone may not cover all of your nighttime energy costs, or offer as much savings as a battery can.
Hybrid systems and solar generators
While solar storage is the most seamless way to keep your home powered at night, it’s not your only option. If your utility’s net metering program is limited, or you want more control over how and when you use your energy, hybrid systems and solar generators are worth considering.
Hybrid systems let you switch between grid-tied and off-grid use of your storage system. They can be a good option if your utility doesn’t offer a good net metering program, or if time-of-use billing is expensive. Solar generators, on the other hand, can be helpful if your area experiences frequent power outages. Compared to battery systems, solar generators are smaller and much more portable.
Conclusion
Solar panels still work in cloudy conditions, but a smart solar system design will help you optimize your output in a range of conditions. And using battery storage can help you take advantage of your system’s peak production at midday to power your home in the evening — and keep the lights on when the power goes out.
But even without a solar battery, your utility company’s net metering policies can help you offset your electricity costs when your panels aren’t producing electricity by contributing to the grid during the day. Either way, a well-designed solar system ensures you get reliable, cost-effective solar power, rain, shine, or even after sunset.
Working with a trusted installer is a great way to ensure that your solar system is designed to meet your home’s specific needs. Aurora Solar makes it easy to connect with multiple vetted local installers and compare quotes all in one place. Get started here.
Frequently asked questions
How do solar panels provide power at night?
During daylight hours, the excess energy from your solar panels can be used to charge a home battery storage system that you use in the evening. A solar-plus-storage system with a 10 kWh battery can power a house that uses 5 kW for about 2 hours. If you don’t have battery storage, net metering lets you send extra solar power to the grid during the day in exchange for energy credits, which you can use to help cover your electricity needs at night.
Is it better to charge my EV at night or during the day?
If you have solar, charging your EV during peak daylight hours can be the cheapest way to power up your car. Plugging in your EV at this time lets you take advantage of the highest power output from your solar panels.
Time-of-use billing generally makes EV charging more expensive in the evening. But in the overnight hours, electricity from the grid is typically at its cheapest, making this the next best time to plug in your electric vehicle.
Can moonlight charge solar panels?
Moonlight only has a fraction of the power of the sun, and it’s usually not enough to start the process of converting photons to electricity in your solar panels. Once your solar system is installed, though, you might see a tiny amount of output on a full-moon night. Unfortunately, moonlight doesn’t offer enough light to power modern appliances in your home, much less your entire home.
