Preparations for the Aug. 21 eclipse show just how important solar power has become to California, as power grid overseers decide how to replace energy lost when the sun's rays are blocked by the moon.

Solar panels generate 20 to 40 percent of our electricity on a typical sunny morning and sometimes as much as 47 percent. But the day of the eclipse, much of that power will be turned off over a period of three hours. While Oregon gets a total eclipse, in LA about 62 percent of the sun will blocked by the moon.

California power officials estimate utilities will need to generate 6,000 additional megawatts to make up for the solar shortfall, enough to power 6 million homes.

Skies will start to darken in Southern California shortly after 9 a.m. on the day of the eclipse. The region will reach the maximum darkness at about 10:19 before returning to full sun about 11:44.

The big issue is how quickly solar power will ramp down and how quickly it can be replaced by other sources.

“What makes this more unique is how quickly this is going to happen,” said Steven Greenlee, spokesman for the California Independent System Operator, which oversees much of the power distribution in the state.

"The ramp-down and then ramp-up of solar, synched with the ramp-up and then down of non-solar resources to cover the gap will be a challenge," Greenlee said.

An eclipse causes a much steeper drop-off in power than a sunset or sudden cloud cover, Greenlee said. A monsoon cloud that comes on suddenly could cause a ramp-down of about 29 megawatts a minute. The eclipse comes on even more rapidly, ramping down at about 70 megawatts a minute.

As the state loses sunlight, people will be turning on lights as if it were night, and homes with rooftop solar will start drawing power from traditional sources, adding more demand to the power grid.

The challenge is like trying to keep the same level of water in a swimming pool while it is draining out on one end and being refilled at the other end, he said.

Utilities will coordinate how to replace the lost solar energy through other sources like wind and natural gas without causing surges or outages.

Power distribution from the Los Angeles Department of Water and Power is not managed by the California ISO. However DWP officials are also preparing for the drop in sunlight, said spokesman Joseph Ramallo.

The other risk is that something unexpected happens to interrupt the flow of power from one of the CA ISO sources of power. An example is a brush fire taking out a transmission line, something that happened last August during the Blue Cut Fire in San Bernardino County. The eclipse is happening in August, which is in the middle of brush fire season in Southern California.

California gets it solar power from plants in the San Joaquin Valley, Mojave Desert, the Coachella and Imperial valleys and Las Vegas. The eclipse will make it darker near the northern solar plants like in the Northern San Joaquin Valley, which will have a 76 percent eclipse, and Las Vegas, where it will be a 72 percent eclipse. To the south, the plants lose less sun.