While oversizing the solar array relative to the inverter‘s rating can help your system capture more energy throughout the day, this approach is not without costs.

 

“Either spend money on an additional inverter or lose energy harvest to inverter clipping.”

 

 

When the DC maximum power point (MPP) of the solar array – or the point at which the solar array is generating the most amount of energy – is greater than the inverter’s power rating, the “extra” power generated by the array is “clipped” by the inverter to ensure it’s operating within its capabilities.

 

The inverter effectively prevents the system from reaching its MPP, capping the power at the inverter’s nameplate power rating.

 

To prevent this, it’s crucial to model inverter clipping to design a system with a DC-to-AC ratio greater than 1, especially in regions that frequently see an irradiance larger than the standard test conditions (STC) irradiance of 1000 W/m2 (higher levels of irradiance lead to higher power output).

 

The US Energy and Information Administration (EIA) states, “for individual systems, inverter loading ratios are usually between 1.13 and 1.30.”

 

 

For example, consider a south-facing, 20°-tilt ground mount system in North Carolina (35.37° latitude) with a 100 kW central inverter. If we design the system with a DC-to-AC ratio of 1, it will never clip; however, we will also not fully utilize the AC capacity of the inverter. We have two options. Either spend money on an additional inverter or lose energy harvest to inverter clipping.

 

Knowing how much energy is clipped allows a designer to understand how effective the oversizing scheme is at increasing energy harvest, and ultimately determine what system configuration is the most cost-effective.

 

The chart below shows three DC-to-AC ratios and their estimated losses to clipping.

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