On August 21, a total solar eclipse will travel across North America -- and put grid operators to the test.

The solar industry’s success means that utilities and grid managers across the U.S. will have to plan for a period of time when the sky goes dark and solar generation plummets. This is especially true in California, where solar makes up 10 percent of the electricity mix and accounts for around half the entire nation’s total solar generating capacity.

For several hours on August 21, the moon’s shadow will trigger a 6,000-megawatt generation shortfall -- equivalent to the power demand of Los Angeles -- according to the California Independent System Operator (CAISO). That net load effect assumes large-scale solar production will drop by an estimated 4,194 megawatts, distributed solar with drop by a lesser amount and that wind production will remain at a steady clip based on historical performance.

Source: California ISO

While the generation loss is significant, it’s not the only factor CAISO needs to handle. The grid operator also has to manage the ramp rate, which will see the eclipse cause solar generation to drop off at 70 megawatts per minute, then ramp up at 90 megawatts per minute as the shadow passes. CAISO’s typical ramp rate is 29 megawatts per minute.

“It will be the first like this, that’s for sure,” said Steven Greenlee, senior public information officer at CAISO. “Grid operators, not only in California, but others across the U.S., are going to have to deal with this. But we’ll probably have to deal with it more because we have nearly 10,000 megawatts of solar resources connected to the grid, of all sizes.”

At two recent speaking engagements, California Public Utilities Commission President Michael Picker called on members of the cleantech sector to help California residents use less electricity during the eclipse.

“I don’t think it’s a huge issue, but I want people here to help me figure out how to reach customers,” he said at a Los Angeles Business Council event. “We can do our California thing. We can mobilize and respond without powering up our peakers, without dragging baseload power plants out of retirement to supplement the grid. We can ask people not to charge their phones, not to turn on their washing machines. We can ask them not to charge their EVs. There are simple things we can do to power that hour and a half.”

Some critics may think the eclipse is when California "falls apart," Picker said. But by showing that the state can handle a grid with high levels of solar penetration during such an event, the state can “send a message to Rick Perry telling him not to worry about California,” said Picker, referring to comments the Energy Secretary made about the need to shore up baseload power resources.

Source: California ISO

Speaking last week at a conference on community choice aggregation -- an emerging trend where local governments take the procurement of electricity generation into their own hands and away from the incumbent utility -- Picker said he wanted community choice leaders to ask their customers not to use electricity during the eclipse hours. He called on local organizers to engage on the issue, as a way to prove the relatively new concept of community choice aggregation can work well with the larger grid system.

“Bring me evidence you can help manage this,” Picker said.



CAISO is looking at managing the eclipse a bit differently. Greenlee said the grid operator isn’t expecting to ask customers to reduce energy use. Instead, it’s planning on securing generation from other sources, specifically hydropower and natural gas, and using them to manage the ramp.

“We’ll be needing to bring on some natural-gas plants to help us with our flexible ramping product to make up for lost solar,” said Greenlee. “So we need to make sure the gas supply is going to be in place and generators have it procured and are ready to generate during that time. Then we also have...plenty of hydro generation this year, so we’re going to be using that as well.”

Source: California ISO

It’s hard to say exactly how much of each resource will be needed to manage that 6,000-megawatt solar shortfall, he added. If it’s not enough, and there is some kind of grid stress, CAISO will issue a flex alert, which is a voluntary call for conservation. Electricity users directly involved in CAISO demand response programs could respond to the alert. Otherwise, investor-owned utilities, which run the bulk of California’s demand response, will trigger their programs. But that would be a last resort, Greenlee said.

While hydropower and natural gas will play a key role in supplementing solar power during the August eclipse, it doesn’t mean baseload power is coming back. The eclipse is actually a perfect example of why the state needs flexible resources and not baseload power, said Greenlee. “Because they can ramp up and down quickly, and that’s what we need it to do,” he said.

This shift isn’t daunting, because it’s part of a transition that’s been taking place in California for roughly 15 years.

“Here at ISO, we have pioneered how to manage the variability of wind and solar resources, and so this is the most that we’re having to manage, but we have a good foundation going into this eclipse,” Greenlee said.

“The old grid was based on baseload power, and most of the baseload was close to the urban centers that were using it,” he added. “Now we have resources that are more remote, farther way and more variable. So it’s a new paradigm, and one we think is very exciting. We’re helping to create the grid of the future.”