How does your location affect the optimal inverter size?

By Paul Grana, co-founder of Folsom Labs

We previously showed that over-power clipping losses are not nearly as high as people typically think. A commercial system in Atlanta with a DC/AC ratio of 1.3 would have just 0.5% of over-power clipping loss. However, that analysis just looked at one location. We find that the results will change significantly based on where the array is located. And that not only will the system performance change, but it can lead to differences in system design from one location to another.

Sunlight (and clipping) varies by location

We chose Atlanta because it was middle-of-the-road in terms of sunlight received. If we look at other cities that are popular for solar, we can see that annual sunlight ranges from over 2,100 W/m² in Phoenix to just over 1,100 W/m² in Seattle.Of course, this isn’t exactly news that Arizona gets more sun than Washington does, but less obvious is the impact it has on over-power clipping losses. To visualize this, we can organize the number of hours spent at each level of sunlight intensity.

The intensity of sunlight has a close relationship to clipping losses. A system clips when the DC power is larger than the inverter’s rated output power, so the number of hours that a system spends at or near its peak output determines how much energy will be lost due to clipping. Note how many hours Phoenix has at peak production—a full 200 hours where the modules are receiving more than 1,000 W/m²! As a result, the clipping losses in Phoenix are nearly twice that of the arrays in Atlanta or Seattle.

Impact on optimal system design

This relationship between sunlight and clipping loss doesn’t just change the energy calculations. It has implications for the optimal system design that an engineer will implement.

For example, if a designer has a target of 1.5% clipping loss, this would result in a 1.3 DC-to-AC ratio in Phoenix, and a 1.45 DC-to-AC ratio in Spokane. A designer in Washington could save ~15% on inverter costs compared to the same designer in Arizona, and maintain the identical system efficiency.

Ultimately, a designer assembling systems in a range of locations is likely to end up with a different inverter-to-array sizing at each location. And the upside of designing in low-irradiance locations like Boston or Seattle is that you can save a bit of money on inverter costs!