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  • Writer's pictureDave Boyer

Solar Capacity Factors

Living in western Pennsylvania for over 20 years, sunshine is revered. Especially the "gray" season from October through May. Moving to a rural area with more frequent power outages, I continue to research options for increased resilience. With no access to natural gas, our home is entirely electric. Heat pumps provide primary cooling and supplemental heating alongside our wood stove. Due to our location, cooling needs are minimal in the summer. Therefore, our biggest electric draw is in the winter. One of the options under consideration is a solar plus battery backup system.

Critical while researching backup systems is understanding the amount of actual electricity generation. This issue leads to one of my biggest frustrations with the renewables market, nameplate capacity.

The usage of solar nameplate capacities is significantly flawed. Publications from news articles noting renewable electric additions to shopping for residential solar panels use nameplate electricity values. To an average consumer, the amount of electricity generated by the system over time is an afterthought.

Considering my situation in western Pennsylvania, I was curious to see how the capacity factor (Capacity Factor = Actual Energy Generated/(Nameplate Capacity x time)) changed throughout 2021. With a peak personal demand and the highest risk of power loss in the winter, I am less interested in the average value typically published.

Unable to find many resources highlighting this type of data, I created maps based on the publicly reported EIA utility data (Data from EIA-860 and EIA-923). First, the capacity factors are calculated every month for each facility in 2021. For each month, I map a rough draft grid and contour lines. The contour lines are revised by hand while checking the data points. Derived from the revised contours, I export each final monthly grid.

June 2021 Capacity Factor
June 2021 Capacity Factor

As expected, there is a significant difference between summer and winter.

In my area, the June capacity factor is approximately 19%. However, in December, the value drops to about 5%.

What does that mean for power generation?

December 2021 Capacity Factor
December 2021 Capacity Factor

If I install a 10kW nameplate solar array and use the capacity factors above over the month:

  • June = 1368 kWh

  • December = 372 kWh

That is a decline of nearly three quarters (~73%).

Reviewing my electricity usage, the peak month was in December at around 700 kWh. So, if I were to install a 10kW array, I'd likely overproduce my summer needs. But, only produce about 50% of my winter usage. With a battery backup and reducing loads during blackouts, this would meet my needs for resiliency. But, I would still be dependent on the electrical grid for regular daily use.

Below are a few other regions across the United States.

  • Area | June CF | Dec CF | % Change

  • Central Montana | 42% | 4.5% | -89%

  • Central Wisconsin | 38% | 6% | -84%

  • Southern Arizona | 44% | 18% | -59%

  • Southern FL | 22% | 22% | 0%

The Northern Plains is one area that stood out. In these higher latitudes, the seasonal shift is very apparent. Summer capacity factors are very high, but the winter values drop significantly.

In contrast, Southern Florida is very consistent throughout the year. Likely due to cloud cover, the summer peaks are lower, but with the lower latitude, the winter remains elevated.

I am sure that utility companies and the EIA have much better models highlighting the changes in capacity factors. But, the general public does not understand these limitations. Data should evolve away from nameplate capacity numbers and show actual power additions.

Other Notes:

  • Capacity factors have improved over time. I did not factor in the installation dates while mapping. However, site location appears more influential than installation time.

  • I completed this analysis in my free time over a few weeks. Data used was as received from the EIA reports. Significant time was not devoted to the QA/QC of the data points. Capacity factors of zero were removed from the data prior to mapping. If anyone is interested in a detailed report, feel free to reach out and discuss.

  • Disclaimer: All data depicted on these maps shall be treated as is for presentation purposes only. Any other use of these maps, or data thereon, is strictly prohibited. Although we strive to provide the best data, outside data sources are used. Mudrock Energy, LLC makes no representation or warranty as to the accuracy of these maps, or the data shown thereon including the identification or location of wells, pipelines, structures, facilities, or lease boundaries. The data is not intended to be used as a surveying product. The maps have been created for nonauthoritative use. The specific locations of such data must be determined by a field survey performed by a licensed surveyor.

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