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Why Is my 10kw system not always producing 10kw?

By Rachel Anderson on May 2, 2020 3:34:07 PM

Topics: Insider

A common and valid QUESTION.

Recently a customer bought a 10kW system and as he monitored its output he noted the MAX generation was 8 KW.  Like many customers he was thinking this was off and there was something wrong with the system. I was able to easily explained to him that this is normal and wanted to share the quick and easy explanation for this difference in name plate system size and actual real time output. Here is how it works and why panels do not generate their rated capacity.

How panel ratings work and what to expect 

The wattage on the panel whether its' 230 watts or even 400 watts is a factory specified capacity as shall function under ideal test conditions in a laboratory called standard testing Library or STL.

Only under those testing conditions will a panel be expected to produce full power at a constant rate.

While panels installed in the field can produce their nameplate output, this is only often momentarily and in some geographic locations not achievable.  Given this, an acceptable range of about 80-90% of the nameplate is a more realistic operating norm when the sun is at the “solar noon” or sunniest part of the day.

If you watch from sunrise to sunset, you will start to see a bell curve. It will kick on as the sun rises and rise slowly through the day to the solar noon. Then you will see it solar decrease as the sun starts to set.

in conclusion should i expect 10kw from my 10kw system 

Probably not more than a few moments each year.  A system will only produce 100% of the “name plate rating” because “real world” is never going to be the same as laboratory testing conditions. If testing conditions is 70 degrees no location in the real world will be 70 degrees all day every day with no rain or cloud ever. This is why you should buy your system based on how many KWH (kilowatt hours) you need to produce to off set your energy bill. It is different for everyone. The other factor is tilt, temp, orientation of the panels as well as location. You can have two neighbors who buy the exact same system. One has their roof orientation as North and South and the other east and west. Those system will produce completely differently even though it is the same system. As you can see there are a lot of factors at play and this is why customers should trust sizing their system to a solar expect who can analyze usage, factor location, azimuth and shading to and help decide what is needed.    

Rachel Anderson

Written by Rachel Anderson

Rachel Anderson is the PE Solar Vice President of Operations and a SEI certified solar specialist. Rachel has 8 years experience directing residential photovoltaic installations and supporting renewable technologies.