Insured losses from southern Californian bushfires could exceed $US1.6 billion ($1.7 billion) as authorities struggle to contain more than 20 separate blazes raging across the state.

As the Californian Department of Insurance declared a state of emergency to speed claims processing, insurers were scrambling to calculate their liabilities from the worst fires to strike the state since 2003.

By late last week, more than 200,000 hectares of land had been burnt out, 1600 buildings razed and nearly a million people evacuated. Another 27,000 homes are under threat, and the fires have claimed 12 lives.

Risk Management Solutions says losses will be at least $US900 million ($973 million) but could reach $US1.6 billion ($1.7 billion) as abundant forest fuel and strong Santa Ana winds stoke several fires burning out of control.

The Insurance Information Institute had previously stated the fires would cost about $US500 million ($541 million) in insured losses. The Witch fire raging in San Diego is alone expected to cost insurers up to $US1 billion ($1.1 billion).

Analyst Goldman Sachs expects losses among the main local insurers - including Allstate, Nationwide and State Farm - to be minimal, but non-mainstream insurance vehicles such as Lloyd's of London could be hardest hit by claims.

Historically the fires will rate as one of California's costliest. In 1991 the Oakland Hills fires accounted for more than $US3 billion ($3.2 billion) in insurance losses (allowing for inflation).

Southern California, including the San Diego area, was hit by bushfires in October 2003 with an insurance claims bill of about $US2 billion ($2.1 billion).

Non-insurance in the latest episode could be up to 20%, based on historical figures.

Fires in Sacramento earlier this year burned out 1200 hectares and destroyed more than 300 buildings. The Californian Department of Insurance said about 78% of residents had insurance cover.