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- How to calculate solar panel power
- How much electricity do solar panels produce?
- How much power do solar panels produce per square meter?
- How much power do solar panels produce per square foot?
- How much power do solar panels produce on a cloudy day?
- How much power do solar panels produce in winter?
- Solar panel power output calculator
Everyone knows that solar panels are rated in watts (Wp), which is the unit of measurement for electrical power. Unfortunately, a 100 watt solar panel may or may not generate that power all the time.
Energy is power in watts multiplied by time and is a more useful way of measuring how much solar panels produce:
Energy in watt-hours (Wh) = Solar power rating (Wp) x Hours
Exactly how much power a solar panel produces depends on several factors, such as title angle and orientation, but by far the biggest influence on solar efficiency is irradiance.
Irradiance is a measure of the amount of the sun’s energy falling on a flat surface and is measure in kilowatt-hours per square meter or peak-sun-hours.
The sun’s power varies with location, but a good average value is 4 peak-sun-hours per day.
How much electricity do solar panels produce?
With an average irradiance of 4 peak-sun-hours a solar panel will normally produce 4 watt-hours (Wh) per day for 1 watt of the panel rated watts (Wp). A standard domestic 100 watt solar panel will produce 400 Wh of energy per day or 146kWh per year.
Manufacturers rate their solar panels according to Standard Test Conditions (STC) which states panel output power in watts with an irradiance of 1000 watts per square meter (W/m2).
The actual out power depends on many factors, but the main thing is how much of the sun’s energy falls onto the panel’s surface. This varies according to geographic location and can vary significantly.
With average irradiance of 4 peak sun hours a 1 square meter solar panel will produce 1460kWh/year.
Table – Compare solar panel power production for cities in US and UK
San Fran., Ca
100W solar panel output per year (kWh)
For the average irradiance value of 4 peak sun hours, solar panel power produced per square foot can be found by the following formula:
- Power produced for 1 square meter / 10.76391
- 1460kWh/year / 10.76391 = 136kWh/yr
Source: Global Solar Atlas
Solar panels continue to produce power on a cloudy day but at a reduced rate of output. Exactly how much is difficult to assess because of the many different types of cloud cover.
Complete cloud cover of dark rain-type cloud would severely reduce power output by up to 90%, while passing clouds in an otherwise clear sky might reduce output by anything between 20 to 50%.
The table below shows the irradiance values for each month for Tampa, Florida. You can see that the lowest monthly output in winter (December) is about 38% of the greatest output in summer (May).
Irradiance and solar power output by month for Tampa, Florida (kWh/m2/day - also are known as Peak-Sun-Hours)
Source: Solar Electrical Handbook
Table – How much power do solar panels produce per day/month/year?
This table shows the amount of energy in kilowatt-hours (kWh) produced each day, month and year for an average irradiance value of 4 peak sun hours.
This irradiance is equivalent to Chicago, USA
Solar Panels (Wp)
kWh per day
kWh per month
kWh per year
So far I’ve shown the theoretical solar panel production, but real-life isn’t like that. A solar installation has losses, both due to design and environmental factors.
These losses can amount to 20% of the overall production capacity and so this calculator uses that figure to give realistic output values of various installations.
Infographic embed code:
<img src=”https://diysolarshack.com/wp-content/uploads/2021/02/10-main-losses-in-domestic-PV-system.jpg” alt=”10 main energy losses in domestic PV system” width=”638″ height=”1200″ />
PDF download of Infographic: 10 main losses in domestic PV system
Go to the Global Solar Data, enter your location by city or GPS, and note the value for direct irradiance (be sure to use the daily irradiance setting.)
It will be between 3 to 6 kWh/m2/day, which is also called the peak-sun-hours and will be used in the calculator to find the power you can expect to be produced by your system
Video – Calculating how much power your solar panels can produce.
How Much Power Do Solar Panels Produce – Web Story
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