Why does my 5.2 kW system not produce 5.2 kW?

So a lot of people ask why their 5.2 kW in solar panels doesn't produce 5.2 kW at the inverter.  Its a good question and one that isn't super easy to explain.

This is a chart of the peak power of a 5.2 kW system:

Here is a chart of another system that I installed in the same area that is 8.5 kW with little or no shading:

Neither system has reached its theoretical maximum of the installed size of the system and never will.  The inverter on the 5.2 kW system has a maximum output is 5 kW AC and the 8.5 kW system has a 7.6 kW inverter.

There are a couple of reasons why the installed capacity (5.25 kW) doesn’t equal the output of the inverter (charts above).  One is inefficiencies of the system (wiring losses, inverter not being 100% efficient, etc.) and the other is that solar panels are rated at what is called Standard Test Conditions (STC).  This is a laboratory test that uses artificial lighting to produce 1,000 watts per square meter, at a solar cell temperature (not ambient air temperature) of 25 degrees celsius, and an air mass of 1.5.  So for a 250W solar panel to produce 250W of power these three conditions would have to occur in the ‘wild’.  This is theoretically possible, but not likely.  You have to also remember that this 250 watts of power will equal something less than 250 watts of AC power (output of the inverter) because of inefficiencies.  

The chart above does highlight the potential spike in the fall and spring that you might see (in this case in November).  The fall and spring are when you’ll see conditions most like STC; low temps with high irradiance (sunlight).

The important number to look at with a solar system is how many kWh it produces in a month or year.  System designers look at all the environmental factors (location on Earth, weather patterns, azimuth and tilt of panels, etc.) to model the estimated output in kWh of the system.

I welcome comments or questions so I can improve on the explaination.