Solar panelsBurning questions

How many hours a day do solar panels work?

Like some people, solar panels wake up with the first ray of the sun and go to sleep when the night falls. Like most people, they can’t work at their 100% for the whole day. That’s why a simple question of how many hours a day solar panels work gets a complicated answer in the form of this article.

Peeking at peak sun hours

There is a difference between working long and working efficiently. Solar panels generate energy from dawn till dusk, but that doesn’t mean they give their all at each moment. There are such things as daylight hours and peak sun hours.

Solar panels are tested at 1000 W/m² irradiance in lab conditions

Daylight hours last from sunrise to sunset. Peak sun hours are the time when sunlight intensity is best for the generation of solar energy. The irradiance levels reach 800–1,000 watts per square meter. This means your 5-kilowatt solar system may generate 5 kilowatt-hours of direct current.

Seattle has about 14.5 hours of daylight in summer and Phoenix has about 13.5 hours. At first glance, solar panels in Seattle seem more hard-working, but far from it! If we compare the average number of peak sun hours in summer, we’ll get 5.38 in Seattle and 7.4 in Phoenix, according to NREL. This means that solar panels in Phoenix generate about 1.4 times more energy than in Seattle.

In winter, the average number of daylight hours drops to 9.5 hours in Seattle and to 10.5 hours in Phoenix. The average number of peak sun hours gets even smaller: 3.6 hours in Seattle and 5.5 hours in Phoenix. The difference in power output between summer and winter may be up to 40–50%.

Calculating capacity

Peak sun hours in your area are a key factor when you calculate the size of a solar system for your home.

First, you need to calculate your daily household consumption. Check your electricity bill and divide it by 30 days. Then divide it by the average number of peak sun hours in your area. You also have to account for 20-30% energy losses from the conversion and transfer of current. 

The average consumption of a medium-sized house in America is about 30 kilowatt-hours per day. Washington has 4.9 average peak sun hours per day throughout the year. Let’s calculate the size of a solar system that would cover the daily needs of a Seattle resident.

30 kWh/day / 4.9 psh + 25% = 7.8 kW

Arizona gets 6.3 average peak sun hours per day throughout the year. Here is the size of a system that would cover the needs of the average house in Phoenix.

30 kWh/day / 6.6 psh + 25% = 5.6 kW

We used annual average values for calculations. On short winter days, you may need to draw energy from the grid, and on sunny summer days, you can sell excess energy and save some money.

Let us do the math for you

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Tweaking performance

Sun hours aren’t the only thing that affects solar panels’ performance. The most obvious one is the weather: on a cloudy day, solar panels work at 60–80% of their capacity. Solar panels also don’t like heat. When their temperature gets over 77°F, the power output starts falling by up to 10%.

The production of your system also depends on how solar panels are installed. In the northern hemisphere, solar panels perform best when they face south. Facing east or west, solar panels produce about 15% less energy. A system turned slightly to the west generates more energy in the evening though. The southeast-facing system absorbs more sunlight from the sun in the morning.

Choosing west or east can be good if you need solar energy at a certain time of day

Years of experience in translation and a love of nature help Julia find the right words to encourage going solar. She joined the team in 2023 and is happy to make her contribution to a greener future.

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