A common question in solar is “Why are you installing a 260 watt solar module on a 215 watt microinverter?” Or for central inverters, “Why is my system a 9,000 watt system on a 8,000 watt inverter?”

Solar modules don’t produce their nameplate (DC) rating even with perfect sunlight that is perfectly oriented to the modules — and even when this is approached, it’s for very limited times in very specific, short-term situations.
When a solar modules nameplate says 300 watts, this means that in perfect conditions the module will produce 300 watts of power. In controlled conditions with a constant irradiance of 1000 W/m2 at 25 degree Celsius or 77 Fahrenheit a manufacturer will measure how much power comes out of the module. This measurement is called Standard Test Conditions or STC for short thus the nameplate rating is known as STC watts.

In the real world, the irradiance is usually lower and the temperature of the module is higher. The module production decreases with increase in temperature and decrease in irradiance. Dirt and soil covering the module will further reduce its production. The characteristics of each module in an array are never rigorously identical. Electrical losses occur due to slight differences caused by manufacturing imperfections, this is called the array mismatch loss.

The temperature loss, irradiance loss, dirt/soil loss and mismatch loss typically reduce your array production by 20%. This is why the inverter is usually sized 80% of your array capacity.

There will be a few days in a year when your array will receive bright sunlight on a cool day. On such days your array will exceed the maximum input power capacity of your inverter and you will experience minimal power clipping on your inverter monitoring as shown below.

power clipping

Figure 10 – Power Clipping

This only occurs a few times and over the short and long-term, driving the inverters to their maximum production maximizes financial return and keeps your up-front costs down by not buying more inverter capacity than is actually needed.

– Thanks to Pranav Modak, Erin Noble, and Dane Glueck for this concise explanation.