When the sun starts to set in California, there’s one thing you can count on: thousands of megawatts of natural gas-fired power plants quickly firing up to keep the state lit.

It’s a daily phenomenon that will become more pronounced than ever this winter as California’s ambitious clean energy goals have boosted the state’s use of renewables. The surge in intermittent solar power will test the statewide electricity grid because it exacerbates the need for alternative sources such as gas outside of daylight hours. Regulators have warned it’ll make California more vulnerable to price spikes and power disruptions.

It works like this: As the day begins to wane in the Golden State, generation from solar panels drops off. That occurs just when consumers returning home from work turn on appliances and flip on lights, driving up electricity consumption. Other power supplies are needed to fill the gap and the need is more urgent in winter when days are shorter.

The phenomenon known as the “duck curve” is so named for the resemblance of the demand slope to the profile of a water fowl. The California grid’s need to call on gas-fired plants to balance shifts in demand and supply shows the potential hazards of tying more renewable generation to power networks.

“This is one of the reasons why the ISO is advocating for flexible resources that have the ability to start and stop, and change output levels up or down several times a day,” said Steven Greenlee, a spokesman for the California Independent System Operator Corp., which manages the state’s grid.

Solar generation accounted for 19 percent of California’s electric supply on Tuesday afternoon, data from the grid operator showed. Large-scale solar projects from utilities have grown to about 7,000 megawatts in 2015, while rooftop panels on houses and businesses now supply as much as 3,000 megawatts, according to the Federal Energy Regulatory Commission.

Storms and cloud cover expected for California this winter could reduce the grid’s need to ramp up gas-fired generation, said Rick Margolin, senior natural gas analyst at Genscape Inc., a Louisville, Kentucky-based market analytics firm.

The “duck curve” probably won’t be as much of a problem this winter because of an expected drop in solar output due to El Nino weather conditions, he said by e-mail.

The gas-fired generation needed to back up solar generation on the system surged to 9,131 megawatts last winter, up 46 percent from three years earlier, FERC said. California ISO expects it will need to rise further to 13,000 megawatts in 2020.

The state has set a goal of getting half of its electricity from renewable sources by 2030.

“Together, the need for gas-fired generation and the lack of dispatchable renewable generation increases the likelihood of price volatility and possible over- and under-generation conditions,” FERC said at its monthly meeting in Washington. The curve “is a particular challenge in the winter when the sun sets well before the evening peak load.”

Spot wholesale power for California ISO’s SP15 hub, serving Los Angeles and San Diego, dropped $2.01, or 8.6 percent, to average $21.40 a megawatt-hour during the hour ended at 10 a.m. local time on Wednesday from the same time a day earlier, grid data compiled by Bloomberg showed.