Do you want to know how much energy does a solar panel produce per square foot? Is this because all the calculations you can find are in square meters? If so you have come to the right place.
To accomplish this feat. We need to know how much sunlight can be absorbed by a solar panel at sea level. This will then form the basis to calculate the wattage it will produce per square foot.
I will also be looking at average daily sunshine and the subject of irradiance and how it can affect solar installations.
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How Much Energy Does A Solar Panel Produce Per Square Foot?
“The amount of energy that falls from the sun on the earth is around 126.4w per square foot. A solar panel can absorb around 92.94 watts per sq. ft at sea level. Assuming a solar panel with a 22.5% PV cell efficiency. Based on the watts produced per sq. ft and the efficiency. The panel can produce around 20.91w per square foot.”
If you prefer your measurements in meters, available sunlight is around 1000w per m² at sea level. Based on a 22.5% solar cell efficiency, the solar panel can produce around 225w per m².
Of course, the efficiency of the solar panel is based on wattage production during standard test conditions (STC). Below is a table laying out solar panel watts per square foot and m² based on their efficiency.
|Solar Panel Efficiency||Watts Produces Per Square Foot||Watts Produced Per Square Meter|
What Are The Sq.ft And M² Wattage Production Figures of Solar Panels Based On?
I am not going to bore you to death with this because this post is meant to be informative, not academic.
But according to NASA Earth Observatory;
At Earth’s average distance from the Sun (about 150 million kilometers), the average intensity of solar energy reaching the top of the atmosphere directly facing the Sun is about 1,360 watts per square meter, according to measurements made by the most recent NASA satellite missions
The average intensity of sunlight at sea level is approximately 92.94w per sq. ft or 1000w per m². Manufacturers base their solar panel wattage ratings on these figures and the efficiency of the panel. But we can only come to an approximation.
Because we don’t know the exact amount of sunlight falling on the solar panel at the time of testing. The manufacturer’s efficiency rating has to be taken at face value. Using the example below we can calculate how many watts per sq. ft it will theoretically produce.
The A-Series Sunpower™ Residential AC Module 420w Solar Panel
This residential solar panel is 20.05 sq. ft (1.86 m²) with an efficiency of 22.5%. To find the approximate wattage output per square foot we need to;
Multiply the assumed amount of sunlight per square foot falling on the panel by the efficiency;
92.94 x 22.5% = 20.91w.
So 20.91 is the wattage the solar panel will theoretically produce per sq. ft.
To find the total wattage per sq. ft produced by this solar panel.
We need to multiply the panel’s physical 20.05 sq. ft size. By the watts per square foot (20.91) the panel can theoretically produce.
20.05 x 20.91 = 419.25w.
So 419.25 is the total theoretical amount of watts the solar will produce
Sunpowers™ officially rated wattage for this solar panel is 420w. As you can see there is less than a watt difference in my theoretical calculation of 419.25w. You can perform this simple calculation with any solar panel.
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How Many Kwh Per Square Foot Of Average Sunshine Does The US Receive Per Day?
Standard test conditions are all well and good. But in the real world, the test conditions are rarely met. There are lots of variables such as:
- Which way your roof faces south or north
- Irradiance (How much sunlight you receive)
- Your latitude relative to the sun,
- The slope of your roof relative to the sun
- The season
- The time of day
- The outside ambient temperature
- The weather conditions
I know it’s a long list, isn’t it? But all these things affect a solar panels’ rate of sunlight collection. So to cut through it all let’s take a look at the average daily sunshine in kWh per sq. ft. If you don’t know there are 1000 watts in a kilowatt-hour (kWh)
The Average Daily Sunshine Per Sq. Ft By Irradiance
Below is a map showing the average daily sunshine collected over 18 years 1998 – 2016. The measurements are in m², not sq. ft so I have provided a conversion table.
The data for the map was compiled by;
Sengupta, M., Y. Xie, A. Lopez, A. Habte, G. Maclaurin, and J. Shelby. 2018. “The National Solar Radiation Data Base (NSRDB).” Renewable and Sustainable Energy Reviews 89 (June): 51-60.
On the left, the table shows the number of hours of average sunshine shown in kWh m² which is 1000. For instance > 7.5 = 7,500Wh. On the right, is the conversion to sq. ft and shows the actual watts per hour.
To convert Kilowatt Hours – KWh//day to Watt-Hours – Wh/sq. ft/day.
I multiplied the table figures on the left-hand side by 1000 and then divide them by 10.76. The reason I divided it by 10.76 is that there are 10.76 sq. ft in one square meter
Example; < 4.00 x 1000 = 4000 ÷ 10.76 = 371.75
|Kilowatt Hours – KWh/m²/day||Watt-Hours – Wh/sq. ft/day|
|> 7.5||> 697.03|
|7.0 to 7.4||650.56 to 687.73|
|6.0 to 6.4||557.62 to 594.80|
|5.5 to 5.9||511.15 to 548.33|
|5.0 to 5.4||464.68 to 501.86|
|4.50 to 4.9||418.21 to 441.44|
|4.25 to 4.5||394.98 to 418.21|
|4.00 to 4.4||371.75 to 408.92|
|< 4.00||< 371.75|
You can see straight away that the average daily sunlight per square foot differs across the country. The southern US received the most, particularly the southwest around California, New Mexico, and Arizona.
With most states in the northeast above 35° Lattitude and east of 95° receiving the least irradiance. Such as New York State and the northern areas of states clustered around the Great Lakes. The west Washington State and northern parts of Oregon also show a low irradiance.
If you live in the small clusters of the country where the map shows <4.0kWh/371.71.75Wh. You could receive up to less than half of the Irradiance than states showing > 7.5KWh/697.03Wh.
How Does The Irradiance Per Square Foot Effect A Solar Installation?
House Logic says;
A typical two-story, 2,300-square-foot house with a medium-pitch roof — has a roof area of about 1,500 square feet. Double that if the house is only one story.
Going back to the example solar panel I used earlier in the article with a square footage of 20.05. You would be able to fit 74 of them onto the typical two-story house. This means you would have a potential total hourly square foot energy collection of 31.02kWh.
Back to reality now, the average American household uses about 29.00kWh per day. There is an average of about 6 hours of full sunlight per day. If you would like to check the average hours of sunlight per day your State receives. You can do that here
Using our theoretical 419.25w per hour panel to cover the 29.00kWh per day 15 solar panels are needed.
During the summer, irradiance isn’t a problem. But from the fall to springtime irradiance per sq. ft is much lower. You can take a look at the irradiance map here; U.S. December Solar DNI Average
This could mean that in the States where the average irradiance is low. You may not get the performance you were expecting from your solar installation. If you were thinking about a stand-alone installation (non-grid-tie). You might want to also want to think about installing a backup generator.
My Closing Thoughts
Square footage does affect solar collection as does average daily sunshine and irradiance.
I am always worried about using lots of calculations in a post. But when talking about square-foot measurements it is unavoidable. I have tried to make them as clear as possible and not too difficult to follow.
My blog is reader-supported, The link above is an affiliate link. At NO additional cost to you, I will earn a small commission if you click through and purchase. Learn more.
If you would like to know more about installing solar panels around your home, especially outbuildings. Take a look at:
I hope you enjoyed my post and have found it helpful. If you have any questions about solar panel wattage production per square foot. Or want to leave your own personal review, please feel free to leave a comment below.