How Much Power Do Solar Panels Produce?
How to calculate solar panel power
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 watthours (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 kilowatthours per square meter or peaksunhours.
The sun’s power varies with location, but a good average value is 4 peaksunhours per day.
How much electricity do solar panels produce?
With an average irradiance of 4 peaksunhours a solar panel will normally produce 4 watthours (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.
How much power do solar panels produce per square meter?
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
City 
Chicago, Il 
London, UK 
Glasgow, UK 
Houston, Tx 
San Fran., Ca 
Irradiance (peaksunhours/day) 
4.043 
2.374 
1.860 
4.253 
5.699 
100W solar panel output per year (kWh) 
147.4kWh 
87.5kWh 
67.9kWh 
155kWh 
208.9kWh 
How much power do solar panels produce per square foot?
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
How much power do solar panels produce on a cloudy day?
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 raintype 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 most effective way to maximize output when shading occurs due to cloud cover is to use MPPT solar chargers or inverters. Maximum Power Point Tracking ensure optimum solar panel power output.
How much power do solar panels produce in winter?
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 PeakSunHours) 


Jan 
Mar 
Apr 
May 
Jun 
Aug 
Oct 
Dec 
kWh/m2/ day 
3.45 
5.39 
6.37 
7.23 
6.72 
5.76 
4.74 
3.25 
100w – kWh per year

126 
196.7 
232.5 
263.9 
245.3 
210.2 
173 
118.6 
Source: Solar Electrical Handbook
Table – How much power do solar panels produce per day/month/year?
This table shows the amount of energy in kilowatthours (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 
100 
0.4 
12.2 
146 
200 
0.6 
24.4 
292 
300 
1.2 
36.6 
438 
400 
1.6 
49 
584 
500 
2.0 
61 
730 
1000 
4 
122 
1460 
2000 
8 
244 
2920 
4000 
16 
488 
5840 
5000 
20 
610 
7300 
Solar panel power output calculator
So far I’ve shown the theoretical solar panel production, but reallife 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.
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 peaksunhours 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