Yahoo said its servers, which are hosted by Santa Clara, Calif.-based Exodus Communications, were hit by the outage. That in turn affected Yahoo's personalized services, including instant messenger.
Exodus could not immediately be reached for comment.
Yahoo did not indicate when the system problems started. Some customers began noticing the outage in early afternoon. Instant messanger service was restored about 6:45 p.m. PT.
"One of Yahoo's service providers was affected by the power outages California is experiencing today which consequently affected the accessibility of a few Yahoo services, including Yahoo Messenger," said Shannon Stubo, a Yahoo spokeswoman. "They are working to quickly remedy the situation and hope to have it resolved shortly."
For years, Yahoo had hosted many of its services with GlobalCenter, the Web hosting unit of communications carrier Global Crossing. But Exodus acquired GlobalCenter earlier this year.
California has been suffering from power shortages since early this year, leading power regulators in the state to call for conservation and institute rolling blackouts. Tight electricity supplies and high demand led to two days of rolling blackouts Jan. 17 and 18 in Northern California and statewide blackouts March 19 and 20.
The California Independent System Operator (CAISO), the keeper of the state's power grid, implemented a Stage 1 emergency notice at 8:04 a.m., indicating that demand was likely to outstrip supply. The emergency was upgraded to Stage 3, the highest level, at 5:01 p.m.
"This week's hot weather will give Californians their first chance to really show what they can do to conserve energy," said Jeff Butler, vice president of operations, maintenance and construction for the utility. "We believe our customers will rise to the challenge, and do their part to reduce energy use during this crisis."
The morning crisis was made more acute by forced outages at four California power plants, which have been shuttered for refueling. Several of the closed plants are expected to open in mid-June, but some are not expected to reopen until late June or later.
For 32 consecutive days earlier this year, California was in Stage 3 emergencies due to lack of energy resources.
Outages affecting Internet service have not been limited to California. AOL Time Warner's America Online in April blamed a brief service interruption on a power outage affecting Northern Virginia.