Update ( 0915 ET): Oncor Electric Delivery, Texas' largest transmission and electric distribution utility, tweeted:

"Due to ongoing record-low temps and generation, @ERCOT_ISO has continued to direct Oncor & utilities across TX to drop power load through maintained controlled outages. These controlled outages are occurring across the state and our entire service territory." 

Oncor also tweeted: 

"At this time, @ERCOT_ISO is unable to predict when grid conditions will stabilize. All customers are urged to be prepared for cont' extended outages. Please also prioritize safety. Warming stations are available in many areas- check online or call 211 for more."

Temperatures across Texas this morning are absolutely frigid. 

At the moment, PowerOutage.US shows 4.307 million customers in Texas are without power.

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Update ( 0854 ET): Texas power grid operator ERCOT, which represents 90% of the state's electric load, tweeted Tuesday morning that "generators to return, renewable output to increase = increased customer restoration." 

Bloomberg published a headline that said the grid operator expects all power to be back on Tuesday evening.