The total eclipse of the sun Monday is expected to prompt fossil fuel generators to ramp up quickly to fill in the gaps as solar power goes dark from Oregon to the Carolinas.

Grid operators and watchdogs say the biggest effects from the loss of solar-generated electricity will occur in California, which represents 40 percent of the nation's solar capacity. Other states expected to be especially hit by the loss of solar include North Carolina, Utah, and New Jersey.

For the most part, the East Coast and parts of the Midwest will rely on their fleet of natural gas power plants to fill in the gaps, combined with the output from coal, nuclear, and even wind farms to stave off the losses from solar.

PJM Interconnection, the federally-overseen grid operator that ensures the lights stay on in 13 states and the District of Columbia, said it doesn't expect any major impact to electric grid reliability, such as brownouts or blackouts, although it is keeping a close eye on New Jersey and North Carolina.

"Certainly, this is an unusual solar event, but as far as potential impacts to the grid, PJM and its members are prepared," said PJM President and CEO Andrew Ott. "While this is an anticipated event, we routinely plan and prepare for unpredictable events or things that can't be forecast far in advance, such as severe storms and heat waves."

PJM said solar power produces less than 1 percent of its 13-state grid, with a mix of rooftop and utility-scale solar arrays that make up 2,000 megawatts and 500 megawatts, respectively, across its territory. But even with a small "reduction in power from the rooftop panels" the result still means "an increase in electric demand on the grid," PJM said in a statement on the eclipse.

That increase in demand needs to be filled by something, which can get dicey especially when demand for electricity is high because Monday will be a hot summer day and people need electricity to run their air conditioners.

The North American Electric Reliability Corporation, the congressionally-chartered reliability watchdog, pointed out in an eclipse white paper it sent out to grid operators the concerns it wants to address during the event to stave off problems that could arise as more solar is added to the grid.

The watchdog organization said it hopes to use the event to prepare for the next eclipse in 2024 when solar energy will represent even more of the grid's total generation. The last total eclipse in the U.S. was 38 years ago on Feb. 26, 1979, when solar energy generation barely existed, if at all.

It wants to use the eclipse to examine what happens when a state loses all its solar capacity. NERC, which enforces mandatory grid standards for keeping the lights on, says one of the issues with solar energy is that not all of it is under the control of the operator or a regulated utility. So, when the sun goes dark when electricity demand is at its peak, say in the middle of a hot summer day, it is not completely known how big of a drop in power will be experienced.

In the nine states with the most solar energy-generating capacity, the eclipse does pose real problems if another resource, such as natural gas power plants, cannot ramp up fast enough to fill in the gaps, NERC said.

The need to meet any sudden increase in demand "would indicate a great vulnerability to the eclipse and ramping concerns," according to the white paper. Ramping refers to the ability of a power plant to throttle up and down with changing electricity demands to stave off a loss in power. Natural gas and some oil-fired power plants are able to do this. And a renewable power plant with the ability to store and inject power into the grid would be able to generate when the sun wasn't shining. But few renewable energy resources have storage capabilities.

The nine states are Texas, California, Nevada, New York, North Carolina, New Jersey, Arizona, Massachusetts, and Utah. NERC tracks Canada and also includes Ontario as an area of concern. In some states, such as Utah, where solar can generate up to 39 percent of electricity at peak times, the grid watchdog says solar energy may need to be disconnected from the grid to avoid the huge anticipated drop in power from the eclipse at peak times. In the summer, peak demand for electricity can typically occur from noon until 8 p.m.

"Provided that the eclipse occurred at the projected peak period, then the ramp rate for a five-minute interval would be difficult to control without standby energy storage systems," the white paper reads. "If no energy storage devices exist in a network, then the recommendation would be similar to lessons learned from Germany's operation of the 2015 eclipse in which utility photovoltaic generators were disconnected from the grid in advance of the eclipse event."

North Carolina is another problem area for filling in the gap left by solar because the state is expected to experience almost a total loss of sunlight. "The top four states that may require advanced system coordination for operations prior to the eclipse, during the eclipse, and after the eclipse are Utah, California, Nevada, and North Carolina," according to the paper. "In some parts of each of these states, a 0.9 obscuration will be observed (Utah has a maximum obscuration of 0.95, Nevada and North Carolina will experience a total eclipse)."

A small sliver of southwest North Carolina will be in total darkness. Most of the rest of the state will experience a 90-95 percent loss of sunlight. Most rooftop solar panels can turn about 14 percent of the sunlight they receive into electricity. So, with 50 percent sunlight, that 14 percent is cut in half to 7 percent.

California, which has rapidly increased the amount of solar power it receives at peak times, is expected to experience the largest drop in electricity. The Golden State comprises 40 percent of the U.S.'s installed solar power, according to experts. An analysis by Bloomberg showed that the eclipse will knock out 9,000 megawatts of electricity, which is the equivalent of nine nuclear reactors going offline and enough electricity to power tens of thousands of households across the country.

For the most part, ground zero for how solar fares will be California. The grid operator there admits the heavy lift but is optimistic they will get past it. "The eclipse presents some grid management challenges for California and the West," said Nancy Traweek, the executive director of system operations at the California Independent System Operator. "However, with detailed planning and engagement among all parties we are expecting no shortage of electricity or reliability incidents related to the eclipse."

One of the strategies it is deploying to manage the shortfall in electricity is a massive statewide public information campaign called Energy Upgrade California. The campaign is meant to show Californians how they can use less electricity, from unplugging devices to using more energy-efficient technologies.

The state's public utility commission will be hosting an eclipse party in San Francisco on Monday to promote the campaign.

The commission will use the event to promote the use of renewable energy and saving energy "in order for California to burn fewer fossil fuels and emit fewer greenhouse gas emissions when the sun is partially eclipsed by the moon in the morning hours on August 21," the commission said.

But the effort to ensure the grid keeps running in the Golden State will likely come from the fleets of natural gas peaker plants that ring many of its cities, such as Los Angeles. The only problem will be constraints on natural gas supplies. Aliso Canyon, a large natural gas storage facility near L.A., has faced a number of problems delivering fuel when needed after a major leak in 2015.

SoCalGas began injecting natural gas into the facility last month in line with orders by the state utility commission "to maintain sufficient natural gas inventories at the field necessary to support the reliability of the region's natural gas and electricity systems."

It advised residents nearby the Aliso Canyon facility that due to increased demand on the system, it may be necessary to begin withdrawing natural gas from the facility.

Meanwhile, climate change groups want to use the eclipse to see how renewable energy fares on the grid and what improvements could be learned for the growth of solar energy.

"With the country moving steadily towards higher levels of solar-powered electricity every day, the eclipse may carry important lessons about managing solar resources in the U.S.," said the group Climate Nexus. "As we transition to a low-carbon electricity grid, preparation for natural events like eclipses will teach us important lessons about managing our electricity grid and ensuring reliability of our electricity no matter the threat."