Today could be one of the most taxing days ever on the California power grid, and it got off to a troublesome start, as thousands are without power in Livermore due to transformer failures.

Plenty of people lost power Monday as this oppressive heatwave we’re under knocked out electricity across northern California — but not because of state-ordered rolling blackouts. KTVU reports that around 40,000 PG&E customers were without power as of 7 p.m. the night of Labor Day due to overheating transformers, with more than 17,000 losing electricity in the East Bay, nearly 6,000 in the South Bay, over 2,000 on the peninsula, and about 1,000  in the North Bay.

The risks of power loss due to extreme heat are most significant in the East Bay. A separate segment above from KTVU detailed Monday’s record-breaking 116 degree temperatures in Livermore, a segment where people were in pretty good spirits, picnicking and playing and fountains and such. But that segment was from 10 p.m. Monday night, and Monday was a holiday, and Tuesday is off to a much more troubling start.

In that same town of Livermore, a couple thousand households didn’t have electricity Tuesday morning, and KPIX reports around 3,500 customers were without power. The evenings have not been cooling off, putting electrical transformers at risk of blowing out.

"Throughout the Bay Area, the main cause of outages that we're seeing is transformer failure because these components are not able to cool down overnight," PG&E spokesperson Mayra Tostado, told KTVU. "We have brought in crews from outside the area to support with restoration efforts. Today, we have  more than 240 troublemen and repair crews."

Many of these transformers are at the top of utility poles, and they’re designed with cooler overnight temperatures in mind. But in this particular heatwave, those cooler overnight temperatures aren’t happening.

"If the temperature stays high overnight, they don't cool down as much and the problem is, that can lead to more overheating and blown transformers,"  Severin Borenstein, board of governors member at the state’s power grid manager the California Independent System Operator (CAISO), told KPIX.

Livermore customers are expected to have their power back by 3 p.m. Tuesday afternoon. But the bigger-picture problem is that we hit peak energy usage at about 5:30 p.m. on a weekday, so the possibility of CAISO-ordered rolling blackouts looms over more than just Livermore. And given the scorching temperatures in the weather forecast, that’s a risk we're likely looking at all week.

Related: SF Launches Heat Mapping Effort to Find 'Heat Islands' [SFist]

Image: LPS.1 via Wikimedia Commons