On August 21st, a total solar eclipse will sweep across the United States. This will be the first total eclipse that is visible across the contiguous U.S. since 1918, and the first total eclipse visible from the mainland U.S. since 1979.
Needless to say, this is a rare event.
Nevertheless, it is one of those intermittent events that will impact solar power output. The California Independent System Operator (California ISO), which manages most of the high-voltage power lines in the state (and in parts of Nevada), has been preparing for this eclipse for more than a year.
Why Planning Is Required
You may ask why there is a need for extensive pre-planning, given that solar photovoltaic (PV) output drops to zero every night, and therefore grid operators and power plant managers already know how to deal with solar intermittency.
There are a couple of things to consider. Although California is south of the path of the total eclipse, the state will experience a partial eclipse ranging from ~90% in the northern part of the state to ~60% in the south of the state. The entire state will experience a partial loss of solar power at approximately the same time — an extremely rare daytime event.
Of course, that’s no different than what happens every evening when the sun goes down. The difference, in this case, is that the eclipse will occur late in the morning when solar PV systems are typically near peak output. It also takes place at a time of year when air conditioning demand is high. Sometimes, during periods of high demand, cloud cover can diminish solar PV output in a particular area, but never across the entire state at once.
What that means is that solar power is going to ramp down rapidly across the entire state during a period of relatively high demand — and at a time when peaking power plants are usually not required to operate.