Power to hundreds of thousands of homes and businesses has been cut off in California this week, affecting millions of people.

Pacific Gas and Electric Co (PG&E) said a forecast of extreme wind and dry weather had created fire danger of an unprecedented scope, prompting it to initiate the largest preventive outage in state history to reduce the risk of wildfires sparked by faulty power lines.

Here are some questions and answers about the outage:

How long will the power be out?

PG&E has estimated the power could be out for as long as five days. It said it can't switch the power back on until its equipment is inspected for damage and repaired.

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Turning off the power, avoiding liabilities

While PG&E and other California utilities have shut off power proactively before a fire, the scope of PG&E's power outage appears unprecedented. The power shutoffs are part of a wildfire prevention plan PG&E released in February, after its equipment sparked thousands of wildfires that killed dozens of people. Southern California Edison starting de-energising power lines in fire-prone areas as early as 2003, something PG&E didn't adopt until 2018. Edison's equipment was blamed for starting 456 fires from 2014 to 2018, compared to the 1986 started by similarly-sized PG&E. But like PG&E, Edison has been hit with lawsuits from wildfire victims who blame the utility for the damage.

Why doesn't PG&E move its power lines underground?

Moving power lines underground can reduce wildfire risk, because fires can be sparked during high winds when branches or animals fall on power lines. But it's pricey.

Across the US, there are hundreds of thousands of above-ground power lines using 19th-century technology.

A better approach would be to figure out which parts of the power line pose the highest risk of wildfire and to bury those, said Steven Weissman, a lecturer at the University of California Berkeley Goldman School of Public Policy. "It still might not be enough unless they dramatically increase the amount of money they're spending, but at least it could reduce the risk," he said.

Despite the cost, some American utilities have acted more aggressively to bury power lines underground to limit casualties and damage after extreme weather. On the East Coast, Florida Power & Light spent US$3 billion ($4.75b) starting in 2006 installing concrete poles, hardening power lines and moving nearly 40 per cent of its distribution system underground.

Experts credit the company's work for reducing the damage when the state was hit with subsequent hurricanes.

The cost of retrofitting eventually gets passed on to the utility's customers, PG&E spokesman Paul Doherty said.

About 41,850km of PG&E's distribution lines are currently underground, but they are still vulnerable to damage from natural disasters, earthquakes and excavation, Doherty said, adding "undergrounding is not a panacea".

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Will anyone pay customers for their spoiled food?

Customers may be able to file a claim with an insurance company if it's worth the deductible. But PG&E says don't bother trying to file a claim with the utility. The company doesn't reimburse customers for losses when the power is turned off for safety.

- AP