PJM Prepares for Total Solar Eclipse April 8

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PJM is planning to handle potential impacts to electricity supply and demand when the total solar eclipse briefly blocks sunlight over its geographical footprint on the afternoon of April 8.

What To Expect

The eclipse “path of totality” is 124 miles wide – entering the PJM footprint northwest of Cincinnati and exiting over three counties in northwest Pennsylvania. This is the trail across the planet on April 8 where the moon will completely block the sun for earthlings for some period of time.

While areas of the states in this path will experience a total eclipse for a few minutes, partial shading of the sky is expected to affect most of PJM’s footprint in 13 states and the District of Columbia over the course of about two hours and 30 minutes.

Noticeable darkening of skies overhead, for example, begins in Cleveland at 1:59 p.m. with the two-minute total sun blockage, or “totality,” expected from 3:13 to 3:15 p.m. 

Potential Effects on Generation

PJM’s advance analysis of the weather, electricity demand and available generation supply for April 8 accounts for various scenarios as the earth falls into the moon’s shadow and helps determine how PJM dispatchers will respond in real time.

Weather is the main determining factor in the production of electricity from solar panels, as solar generation generally decreases in direct proportion to the presence of cloud cover. The eclipse will occur when solar radiation is at its highest, Michael Stewart, PJM Sr. Engineer – Load Forecasting, told PJM stakeholders in a presentation at the March 7 Operating Committee meeting.

“The amount of generation we lose will depend on the weather pattern that day,” Stewart said. That is, if it is already cloudy and raining, solar generation will be low to begin with, and the impact of the moon’s shadow will be minimized. If it’s a cloudless day, solar generation will fall by a greater degree, while demand for electricity, or load, could go up or down depending on the temperature.

Even under cloudy skies, PJM is preparing to observe temporary losses of at least 80–85% of the production from the approximate 8,200 MW of grid-connected solar or metered solar resources that are part of the PJM solar generation fleet in early April.

PJM is also preparing for the potential temporary loss of up to 4,800 MW of non-metered or behind-the-meter solar resources during the eclipse. Behind-the-meter resources include residential and commercial rooftop solar generators that are not controlled by PJM’s dispatchers. Decreases in behind-the-meter solar increase the demand on other generation resources in the PJM fleet.

Possible Impacts to Load

As the sun dims, the earth briefly cools, with anticipated dips in air temperature from 4 to 10 degrees. Should conditions that Monday be cool, a temperature dip may drive demand for heating slightly higher. If that weekday afternoon is unexpectedly warm, the eclipse may provide a cooling effect that temporarily decreases the use of air conditioning and drives load down.

Overall, PJM does not anticipate a traditional holiday load pattern, when electricity use tends to be less than normal for the day of the week on which it falls. However, during the eclipse, load is expected to fluctuate based on widespread school closings and atypical weekday travel, with an influx of tourists to some areas to watch the event.

School districts in some areas directly in the path, like Ohio, are closing on April 8 both to avoid safety issues at dismissal time, when the eclipse will be in full effect, and to allow their students to fully experience the rare event with their families.

With a close eye on weather forecasts, PJM is preparing to dispatch generation as needed to respond to solar power losses, including reserve and regulation resources as required. Reserve resources can provide needed backup generation when called upon, while regulation resources can provide energy to help control voltage and frequency on the system.

PJM will provide an update on the anticipated impacts of the eclipse at its April 4 Operating Committee meeting, when the weather forecast for April 8 will be clearer.

The map below from NASA shows the path of totality, umbra, for April 8. Read more about the eclipse on the NASA eclipse page.