Hawaiian Electric fully restored power to customers in East Honolulu on Monday, but not before four public schools had been closed for the day and many customers faced water supply issues.

The company also continued to advise customers on Hawaii island that rolling blackouts were possible Monday after initiating 30-minute outages for 21,557 customers on parts of that island over about an hour Sunday evening because of low power generation.

On Oahu the utility announced at 10:45 a.m. Monday that it had restored power to an estimated 1,353 customers in Hawaii Kai who had been without power since Sunday afternoon.

The Sunday outage, according to Hawaiian Electric, was caused by a 46-kilovolt main transmission line high in the Koolau Mountains falling on a second transmission line amid rain and Kona wind.

Normally, one line is used for service, and the other serves as a backup. But about 4,000 feet of one line fell across the other, taking both lines out of service and cutting power to about 13,000 customers Sunday afternoon.

Hawaiian Electric crews, working without a helicopter because of the weather, were able to cut part of the downed power line so a section of the other line could be reenergized. The initial repair work resulted in power being restored to about 11,000 customers by 9:45 p.m. Sunday.

Another estimated 1,353 customers remained without power Monday morning, leading the state Department of Education to cancel classes at Kaiser High School and Kamiloiki, Hahaione and Koko Head elementary schools.

Also because of the continued power outage Monday morning, the Honolulu Board of Water Supply asked all water users east of Luna­lilo Home Road, including Portlock, and above Hawaii Kai Drive to conserve water because normal pumping equipment supplying one reservoir lacked power to operate.

BWS deployed mobile pumping equipment to the affected Koko Head reservoir, but it couldn’t transport the same amount of water as the regular station pump. So the agency asked customers to conserve water and warned that some customers could lose water service or see reduced water pressure.

With power restoration the water supply issue was resolved, and the four schools were to reopen today. Still, the utility said longer-term work remains for repairing the fallen line so that the area has backup power distribution.

On Hawaii island the utility announced Monday morning that it might initiate rolling outages in the morning and also possibly in the evening due to a generation shortfall and lower than normal wind and solar resources.

“The brief emergency outages may be initiated around the island starting as early as 6 a.m. to prevent loss of power to an even greater number of customers,” said the company, which serves about 90,000 Big Island customers through affiliate Hawaii Electric Light Co.

Customers also were asked to conserve electricity for the rest of the day Monday and especially from 5 to 9 p.m.

On Sunday night, 21,557 customers on Hawaii island experienced 30-minute outages between 8:54 and 9:59 p.m., Hawaiian Electric reported.

The company said a generation unit at its Hill power plant in Waiakea tripped offline Saturday. At the same time, the biggest power plant on the island, Hamakua Energy, was down due to mechanical issues, while generating units at three other power plants were undergoing repairs or had not yet returned to service after an annual overhaul.

There have been rolling blackouts on Hawaii island twice previously this year — once in January and once in February. On March 25, Hawaiian Electric issued a plea for customers to reduce or shift electricity use to help avoid forced outages through mid-April when a generation unit was expected to be returned to service after an annual overhaul. Now the unit isn’t expected to be back online until the end of April.