Here’s one problem with solar power: on August 21, a solar eclipse will throw a shadow over a 70-mile-wide path crossing from Oregon to South Carolina, that may cause a loss of 9,000 megawatts of solar power, the rough equivalent of nine nuclear reactors.

On Thursday, PJM Interconnection LLC, which supervises the nation’s largest power grid covering parts of the eastern U.S., estimated as much as 2,500 megawatts of solar generation could be lost August 21 between 1:30 p.m. to 3:40 p.m. North Carolina and New Jersey may be especially hard hit.

According to NASA, the last time the contiguous U.S. saw a total eclipse was in 1979. Tom DiCapua, managing director at Con Edison Energy in Valhalla, New York, warned, “If it is sunny and all of the sudden the eclipse comes through, there may be a pricing spike in real-time” power trading.

Dave Quinn, a power market analyst at Genscape Inc., noted that the eclipse stands to dim solar radiation by roughly 70%.

The reason solar eclipses don’t happen every month is that the moon's elliptical orbit is tilted more than five degrees to the Earth's orbit around the sun, so its shadow at a new moon usually misses Earth. No more than two total solar eclipses can occur in one year. Total solar eclipses are rare because totality exists only along a narrow path on the Earth's surface traced by the moon's shadow. Total solar eclipses recur at any given place only once every 360 to 410 years on average.