How do home solar panels work at night, on cloudy days, or under snow?

Solar panels make electricity from the sun. So how does your house stay powered when there’s no light? Learn more about how your solar system works 24 hours a day.

An image of the night sky.

Solar panels work by collecting sunlight and magically turning it into electricity to power your home. So does that mean when the sun goes down your electricity turns off and you can’t watch Game of Thrones? Actually, no. Read more to learn about how electricity continues to work seemlessly in a solar home, even when it’s dark, cloudy, or your panels are covered in snow.

First, an explanation. Sunlight is made up of elementary particles called photons. When a photon hits a solar cell, it’s absorbed into the material of the solar cell. But photons carry energy, so when this happens, the cell kicks out an electron in response to that added bit of energy. (Electrons, as you may remember from high school science, are the subatomic particles that form electricity.) A steady stream of photons hitting the solar cell results in a steady flow of electrons out of the cell into the wiring of your solar panel, where they eventually make it into your house.

Do solar panels work at night? Do I need batteries?

No. Solar panels only work when the sun is shining. Without a steady flow of photons, your solar panels do not produce any electrons, which means no electricity will be generated. However, that doesn’t your solar home will be in the dark when it’s nighttime. The vast majority of homes with solar panels remain connected to the electric grid, so when your panels don’t produce electricity, your house will automatically get its power from the grid. You won’t notice anything when this happens, and you don’t need batteries for any of this to work.

This is what happens when the sun is shining. But what happens what it’s dark?

How does a solar house work at night?

I recommend reading the complete guide to solar for your home for background, but here’s a quick overview of the three major components of a home solar photovoltaic system.

We’ve already mentioned solar panels (also known as modules), which are the hero of your system, doing the magical work of turning sunlight into power.

Next is your power meter which, in a grid-connected home, tracks how much electricity you use from the grid, and how much excess solar electricity you send back into the grid. At the end of the month, your utility checks how much power you used and how much you generated, and either bills you for the difference or credits you for any excess.

Finally, the inverter takes the direct current (DC) electricity that your solar panels generate and turns it into the alternating current (AC) electricity that your home uses. If you want to know more about the different types of inverters, you can read our guide on how inverters work.

Your inverter is the key to powering your solar home at night

The inverter is the brains of the operation, and it’s the component responsible for seamlessly powering your house whether it’s sunny, cloudy, or nighttime. To explain how it works, let’s go over some different scenarios.

Scenario 1: Your solar panels generate more electricity that your house is using

Let’s say it’s a bright sunny spring day. It’s not very hot so your air conditioner, which is a big energy hog, isn’t running. But cool spring days are the best time for solar energy generation, so lots of electrons are pouring out of the panels on your roof, and you’re producing more electricity than you need.

A perfectly sunny, cloudless day with solar panels will look like this on your monitoring system: Example solar power generation graph on a sunny day.
A perfectly sunny day with solar panels.

Those electrons flow down the wires into your inverter, which turns that DC power into AC power. From the inverter, the electricity then flows into your electric panel - this is something every electrified home has, whether or not it’s a solar home. It’s the thing with circuit breakers, and it’s responsible for distributing electricity throughout your home.

But if your house isn’t using all of that solar electricity, your inverter knows what to do. The inverter has a connection to the electric grid, over the same wire that you already get your utility power from. In this situation, your inverter routes the excess solar electricity back into that grid connection. Your power meter will spin backward so that you get credit for this power.

Your solar electricity that you sent back into the grid actually gets used by your neighbors, so none of it gets wasted.

Scenario 2: Your solar panels are generating electricity, but your house is using more

If it’s not very sunny, or if it is but you are using a lot electricity (like running your central air conditioner on a hot day) your solar panels might not produce enough electricity to fully power your house.

A partly cloudy day might look like the graph below. In this example, you can tell that it was a perfectly sunny day until the clouds started rolling in shortly before 4 pm:

An example of solar output when clouds start to roll in
An example of solar output when clouds start to roll in.

In this case, your inverter will send your solar electricity into your home but also draw enough power from the utility grid to keep your house powered. It does this for you automatically, and it’s completely seamless. You won’t notice any power spikes or lights flickering when it happens.

If it’s a dark, heavily overcast day, your system output would look something like this:

Solar output when it’s heavily overcast.
Solar output when it’s heavily overcast.

On a day like this, you’ll get a little solar power and draw most of your power from the grid.

Scenario 3: Your solar panels are generating zero electricity

Eventually, the sun goes down and your solar panels stop producing power. When this happens, your house becomes just like any other house on your block that is fully powered by the electric grid. This happens every day when the sun goes down, and you can see it clearly in the graphs above where they tail off to 0 Wh.

But I want to run my house completely on solar power, and this sounds like I’m not.

Some people choose solar power because it’s really important to them to be completely reliant on renewable energy and not contribute to the use of fossil fuels. But the way I’ve described how a grid-connected home normally gets power both from solar and from the utility makes it sounds like even if you are supposedly 100% solar, there are times every day when you will be using electricity from fossil fuels.

Technically, this is true. When the sun goes down, the power to run your tv during Game of Thrones comes from the utility, and anywhere in the the United States in 2019, at least some of that electricity comes from fossil fuels.

But that’s an incomplete understanding of how the power grid works. Power usage across the grid isn’t the same during all times of day. It’s higher during the day when businesses and homes are active, and drops to a low point in the middle of the night when most people are sleeping. During those times of high demand, power plants called peaker plants are dedicated to ramping up generation to supply the electricity to meet that demand.

Peaker plants are powered by fossil fuels - usually, with natural gas - which means that when you blast your air conditioner during a hot summer day, you’re putting more demand on the utility, which responds by asking those peaker plants crank up gas (literally). The example below is a graph of real power demand across New York state for one day. You can explore this yourself in real-time on the NYISO website.

Here’s what actual power demand on the grid looks like. This example is one day of real power usage across New York state.
Here’s what actual power demand on the grid looks like. This example is one day of real power usage across New York state.

But if you have solar panels on your roof, the solar electricity reduces your demand on the grid, or even places a negative demand on the grid by supplying excess power that your neighbors can use to run their air conditioners. The net effect of this is that your panels helped the utility avoid those peaker plants from having to spin up harder.

So you can see that even though you may be using electricity from fossil fuels at night, it’s balanced out because your solar electricity reduces load on the utility during the day. If the environment is one of your top reasons for going solar, you can rest assured that it really does contribute to cleaner air.

The importance of net metering

All of this assumes one thing: that your home has a two-way connection to the grid. Most solar homes do, but the financial model that utilities use to credit you for your excess solar electricity can differ. There are two models: net billing and net metering. Net metering is the more favorable one, because it gives you full credit instead of a partial credit that you get under net billing. To learn more about this, read my article “What is net metering?

Can solar energy be stored and used at night?

Solar panels by themselves to do not store any electricity. The electricity they generate is used immediately. If you really want to make sure that you only ever use solar power, you’ll need a home battery system to store the electricity that you generate during the day. In the evening, when your power production tails off, your inverter will pull power from your battery bank instead of the grid.

This can be expensive, but the costs of batteries are going down. Most manufacturers of home batteries, like Tesla, use lithium-ion technology. This is the same type of battery used in your cellphone, laptop, and electric cars.

In certain cases, there is an additional environmental benefit to this. As renewable power becomes a larger portion of the energy supply, batteries will be required to store power when the wind isn’t blowing or the sun isn’t shining. Batteries help flatten out those spikes in energy generation, and can take the place of traditional fossil fuel peaker plants. This is already becoming important in places where there is a very high amount of solar power, such as Hawaii.

How well do solar panels perform on a cloudy day?

Solar panels continue to produce electricity on a cloudy day, just not as much. Cloud density can vary considerably, from thin cirrus clouds to dense cumulonimbus clouds associated with thunderstorms. Depending on the type of cloud cover, you can expect your solar panels to produce anywhere between 50% to 10% as much electricity in comparison to a sunny day.

Different solar panels are able to cope with cloudy conditions better than others. If you haven’t read my article on how to make sense of solar panel specifications, go ahead and do that. It’ll give you the background necessary to read the datasheets that solar panel manufacturers publish about their products.

If you’ve already read it, you’ll know that there are two common standards for reporting solar panel performance in good light, and they use either 800 W/m² or 1,000 W/m² of solar irradiance (light intensity).

But panels perform differently under dim light. Unfortunately, there is no established requirement for reporting this, but many solar panel datasheets will provide this data. Sometimes it will be in a table, and sometimes they’ll use a graph. For example, here is a Sunpower datasheet for how their products perform at 200 W/m², which is 1/5 the light intensity of the STC standard:

Sunpower specifications for low light conditions.
Sunpower specifications for low light conditions.

Depending on the model, these Sunpower panels produce between 46.2 and 69.5 watts under low light conditions.

Here’s another example. This is from the specification sheet for the Canadian Solar HiKu panel.

Canadian Solar I-V graph.
Canadian Solar I-V graph.

Unlike the Sunpower sheet, this one doesn’t provide performance data in terms of wattage, but an I-V curve, which shows current (I) and voltage (V) under different conditions. The graph on the left shows how power output (in amps) changes according to different light conditions. (The graph on the right shows how it responds under different temperatures.) You can see that Canadian Solar claims basically a linear relationship between light intensity and power output. This is a little less useful than the table that Sunpower provides, but it at least gives you a basis to compare between different solar panel models.

How well do solar panels work when it’s snowing?

Solar panels actually work better in the cold because lower temperatures mean lower electrical resistance. But if it’s snowing, you’re going to have heavy cloud cover and not much light. If your solar panels are covered in snow, you might get no power output even on a sunny day. In my experience, it takes about 2 inches of snow to completely block light from reaching your solar panels.

Contrary to popular belief, solar panels don’t produce heat on their own, so they won’t melt the snow. You have to wait for the temperature to rise and the snow to slide off. Solar panels are dark and do heat up in the sun, so if your panels are only partially covered in snow, sunlight might heat the panels up enough to cause the rest to melt off, even in below freezing conditions.

If you want to get as much electricity as possible, you could use a brush to manually clean the snow off your panels. But this is only a good idea if you can reach them easily without risk of damaging the glass. The little bit of extra power you might generate by doing this is nowhere near worth the cost of a repair.

Bottom line

You can see that your solar powered home works in all kinds of conditions, and you can genuinely claim that you have a solar house, even at night. Batteries aren’t required to make this work, but if you’re intent on going with a battery system or even the extreme of going completely off-grid, your best bet is to consult with a local solar installer who can advise you on the technical aspects of your project and give you pricing. For that, I suggest using our free service to get a solar quote. It’ll connect you with qualified professionals who will provide you with a customized quote for your project.

TAGS:
#Panel Efficiency #Batteries #Net Metering

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