Most comparisons of current solar capacity rely on the raw figures for installed solar without taking into account the size of the area in which that wattage is contained. Calculating the density of installed solar, measured in kilowatts per square mile, reveals true solar hotspots around the county. The results may surprise you.

While Washington, D.C., recently received a good deal of attention for its favorable solar renewable energy credit (SREC) market conditions, it isn’t typically known as a leader in installed capacity. In most rankings, D.C. falls below the top 20 with only a few megawatts of solar capacity. However, when those numbers are converted into kilowatts per square mile, D.C. comes out on top. While New Jersey is a close second, they both command the rest of the field, each with about six times the installed capacity of the third place state. Perhaps D.C.’s dominance shouldn’t be a surprise, since it has a much smaller area featuring mostly urban development without huge tracts of land to dilute its solar density. Nevertheless, it is a useful exercise to demonstrate the commanding solar presence that has been steadily growing in D.C. over the last few years.

In a surprising twist, solar leader California did not rank as competitively compared to the top states. It places not third but fourth, trailing Delaware with under 7kW of solar per square mile. However, California does boast several counties more concentrated than D.C. But it is density leader New Jersey that claims the most concentrated hot spots, ranking off the charts in Hudson County and beating California in several other areas.

Hudson County, with a whopping 563 kW/mi2, borders Manhattan yet far outperforms the tiny island, which features only 18 kW/mi2. In fact, the entire state of New Jersey still outperforms Manhattan. Washington D.C., when compared to all New Jersey counties, ranks just after the garden state’s top three. Among all of the top counties drawn from the highest-ranking solar states, only New Jersey and California contain areas with solar densities that trump the District’s.

Even compared to California, the unquestionably dominant state in terms of installed capacity, D.C. matches up surprisingly well. From 2007-2008, the DOE chose 25 U.S. “Solar American Cities” in which to promote solar technologies and remove market barriers to solar development. Six of those were in California. Without similar directed development but with a strong SREC market, D.C. has surged in installed capacity, now sitting in the middle of those six California cities in terms of solar density. Typical comparisons may not mention D.C. and California in the same breath, but in reality the two are quite close.

As long as installed capacity for solar is measured with no relation to area, D.C.’s progress will not register in most state or city comparisons. However, in relation to its size, its growth in recent years has been astonishing. With a strong SREC market and competitive pricing options, it is likely that this trend will continue into the future.

A note on the data: Installed capacity data was taken from the NREL OpenPV Project and from PJM-EIS GATS reporting. The NREL OpenPV Project relies on open-source data and user contributions to form its database. This data is tentative and imperfect, but still proves useful as a ranking mechanism for states and cities. Until accurate 2011 data is compiled and released for further analysis, we look to this comparison as a strong indicator of D.C.’s strength as a solar market compared to the most competitive areas in the county. GATS data, not available for all states, was substituted when it was higher than the NREL data, giving what we believe is a more accurate picture of total installed capacity. The states for which GATS figures for total installed capacity exceeded NREL include: DC, DE, IL, MD, NJ, OH, PA, VA, WV.

Special thanks to Colin Murchie, Director of Government Affairs at SolarCity, for initiating this conversation.

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