Why Doesn't My Solar Panel Deliver Rated Output?

This is an oft heard complaint coming from solar panel owners.  The answer is not that simple but the most obvious one is due to geographic location and is rarely mentioned by solar panel vendors.

Scientists have determined that a square meter of   the earth’s surface is illuminated by 1000 watts of sunlight.  However this is only true when the sun is directly overhead such as close to the equator. Due to the axial tilt of the planet   the sun may be directly overhead even 23 degrees north or south of the equator depending on time of year.

In North America the industry decided to establish common standards so the consumers can better compare products. The test labs for doing these comparisons are located in Southern California in the desert away from urban pollution and atmospheric contamination that would skew the results.

Unfortunately not everyone lives as far south as this test lab facility.  As a result the slant range of sunlight reaching the installed solar panel has to pass through more atmosphere than sunlight coming from directly overhead\

This atmosphere often  contains  dust particles and thin cloud layers  which  attenuates   the  strength of the sunlight  and thus  the  maximum possible  output  of the    solar panel.

In California the common bench mark for a 1000 watt panel is a total of 1900 kilowatt hour output per year.  This same panel will only produce half as much total power at latitude 53.North.  You can find and download something called insolation charts.  These charts show how many hours of effective useful sunlight for given geographic locations. Some locations are more prone to cloudy weather than other locations.

A common misconception about these charts being that maximum output is still whatever the rating of the panel is.  These charts simply provide you with the number of hours during which the panel put out the maximum possible watts AT THAT LATITUDE  AND  GEOGRAPHIC LOCATION.   An assumption is made that when the data, is collected  and averaged the numbers will be consistent from year to year. 

Climatologists have  noted that flight paths  which concentrate  the  tracks of aircraft  leaving contrails  in the  sky have a measurable  reduction of sunlight reaching   the  ground . This  should also be taken into consideration. 

The  rough rule  of thumb  for sizing  panels  should  be to derate  in proportion to  how far  from the equator you are.  At latitude 49 expect roughly half  as much output.

Below is a graphic illustration to show how it works

In the diagram the sunrays are shown hitting a meter square panel at right angle. The 45 degree angle represents the angular strike. The red line is the same meter wide panel but only 70% of the same band of sunrays hit the active surface.   At latitude 60 the portion of the same bundle of sunrays hitting the active portion of the PV panel is down to 50%.

 This diagram is representative and does not make correct allowance for curvature of the planet nor atmospheric attenuation. This is going to increase losses.

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