Winter storm kills 23 across the country and 3.5m freezing Texans STILL have no power: Gov. Gregg Abbot demands investigation into energy company that may not be able to restore heat for days

Houston, Texas: View from the First Ward neighborhood on Tuesday. The power breakdown sparked growing outrage and demands for answers over how Texas — whose Republican leaders as recently as last year taunted California over the Democratic-led state's rolling blackouts — failed such a massive test of a major point of state pride: energy independence

Richardson, Texas: Shaemiya Taylor, left front, and Marsha Williams, right front, play a board game as Jeremiah Murphy, left rear, and Khloee Williams, right rear, look on at a warming shelter Tuesday. In cooperation with the cities emergency management center, this location is one of seven that have opened in the city, offering those in need a place to keep warm

Houston, Texas: Freezer sections are closed off in Fiesta supermarket on Tuesday. Winter storm Uri has brought historic cold weather, power outages and traffic accidents to Texas

Austin, Texas: People walk on snowy streets Tuesday. Temperatures dropped into the single digits in the state Tuesday 

Governor Greg Abbot has demanded investigation into grid manager, the Electric Reliability Council of Texas

The cold spell has already pushed snow cover to an all time high across the 48 states in North America. Official data shows snow currently covers 73.2 per cent of the area with an average depth of 6 inches; a year ago just 35.5 per cent was covered with an average of 4.6 inches of snow. 

Several cities had record lows: In Minnesota, the Hibbing/Chisholm weather station registered minus 38 degrees Fahrenheit (minus 39 degrees Celsius). Sioux Falls, South Dakota, dropped to minus 26 Fahrenheit (minus 26 degrees Celsius). 

Utilities from Minnesota to Texas implemented rolling blackouts to ease the burden on power grids straining to meet extreme demand for heat and electricity.  

Anger over Texas' power grid failing in the face of a record winter freeze mounted Tuesday as millions of residents in the energy capital of the U.S. remained shivering with no assurances that their electricity and heat — out for 36 hours or longer in many homes — would return soon or stay on once it finally does.

'I know people are angry and frustrated,' said Houston Mayor Sylvester Turner, who woke up to more than 1 million people still without power in his city. 'So am I.'      

Amber Nichols, whose north Austin home has had no power since early Monday, said: 'We're all angry because there is no reason to leave entire neighborhoods freezing to death. This is a complete bungle.'

Nashville, Tennessee: A snow removal vehicle at Nashville International Airport on Tuesday 

Chicago, Illinois: Jennifer Evans stands beside her car, which was damaged when the building collapsed during the storm 

Chicago, Illinois: An aerial photo shows the Chicago skyline and Lake Michigan after an overnight snowfall left more than 18 inches on the ground and roadways

Gov. Greg Abbot has demanded investigation into grid manager, the Electric Reliability Council of Texas

The number of outages in Texas at one point exceeded four million customers.

'This is unacceptable,' Gov. Abbott said, 'Far too many Texans are without power and heat for their homes as our state faces freezing temperatures and severe winter weather.'

He added: 'I have issued an executive order to review the preparations and decisions by ERCOT so we can determine what caused this problem and find long-term solutions.'  

By late Tuesday afternoon, ERCOT officials said some power had been restored, but they warned that even those gains were fragile and more outages were possible.

The grid began preparing for the storm a week ahead of time, but it reached a breaking point early Monday as conditions worsened and knocked power plants offline, ERCOT president Bill Magness said. 

Some wind turbine generators were iced, but nearly twice as much power was wiped out at natural gas and coal plants. Forcing controlled outages was the only way to avert an even more dire blackout in Texas, Magness said.

'What we're protecting against is worse,' he said.

Houston, Texas: People select shirts and sweatshirts being given away at a Gallery Furniture store after the owner opened his business as a shelter for those without power at their homes Tuesday

Houston, Texas: More than 4 million people in Texas still had no power a full day after historic snowfall and single-digit temperatures created a surge of demand for electricity to warm up homes unaccustomed to such extreme lows, buckling the state's power grid and causing widespread blackouts. Those without power in Gallery Furniture on Tuesday 

Houston, Texas: The winter storm has resulted in people sleeping in their cars and furniture stores to keep warm amid unprecedented rolling blackouts that have plunged five million into darkness. Natalie Harrell holds her sleeping daughter, Natasha Tripeaux while sitting in a recliner at a Gallery Furniture store

Houston, Texas: The deep freeze that has paralyzed Texas by knocking out its power grid and sparking an energy crisis saw 5 million homes plunged into darkness amid unprecedented rolling blackouts. Pictured above is homes in Houston without power but empty offices still lit up

 The Federal Emergency Management Agency said Texas had requested 60 generators and that hospitals and nursing homes would get priority. 

Shelters were opened to accommodate more than 1,000 people around the state, FEMA said during a briefing. But even they weren't spared from the outages, as Houston was forced to close two on Monday because of a loss in power.  

A Texas furniture store owner even opened his business as a shelter for those left without power.  

Jim 'Mattress Mack' McIngvale told ABC 13: 'The cold is bitter, so we're opening up the doors to Houstonians. Whether they want to stay for two hours until their power gets back on, or they want to stay for two days, we're here for the community.'     

The Electric Reliability Council of Texas, which oversees the state's main power grid, is still struggling to restore power after failing to keep up with heightened demand. 

ERCOT provides electricity to about 90 percent of the state. 

Images showing empty office buildings in downtown Houston still lit up overnight has sparked outrage given the millions elsewhere without power, while hundreds have been forced to line up outside grocery stores in the freezing cold for supplies.

ERCOT CEO Magness they are 'trying to get people's power back on as quickly as possible' but to do that they 'need to be able to safely manage the balance of supply and demand on the grid'. 

He added: 'The number one job of everybody here at ERCOT is to get people's lights back on. We're seeing demand in the winter nearly like we see at the top of the summer, when we're all using our air conditioners.

'We have seen nothing like this honestly in Texas, that has covered the state like the storm has. It increased demand to an extreme, extraordinary height, and then the storm also made it difficult for the supply to be provided.' 

The spot price of wholesale electricity on the Texas power grid spiked more than 10,000% on Monday, according to data on the grid operator's website. 

Real-time wholesale market prices on the ERCOT power grid were more than $9,000 per megawatt hour late Monday morning, compared with pre-storm prices of less than $50 per megawatt hour. 

ERCOT can be more susceptible to wholesale price spikes because it does not have a capacity market, which pays power plants to be on standby during peak demand and weather emergencies, for example. ERCOT's model means consumers are not paying for generation that may never be called into action.

But early on Monday, ERCOT said extreme weather conditions caused many generating units – across all fuel types – to trip offline and become unavailable. That forced more than 30,000 megawatts of power generation off the grid, ERCOT said in a news release. 

The cold blast caused by winter storm Uri has wreaked havoc on the energy industry with Texas oil wells and refineries halted and natural gas pipelines and wind turbines frozen. 

Experts say the energy crisis essentially boils down to equipment freezing because power plants failed to properly winterize their hardware.  

Oil production in the country's largest crude-producing state has plunged by more than two million barrels a day due to the storm, which has sent prices surging to $60 a barrel for the first time in a year. 

Wind turbines, which account for a fifth of the state's energy, have frozen solid as temperatures plummet to a bitter -20F. 

Texas's grid operator and the Southwest Power Pool, a group of utilities across 14 states, imposed unprecedented rolling blackouts because the supply of reserve energy had been exhausted. Some utilities said they were starting blackouts, while others urged customers to reduce power usage, in a bid to prevent the collapse of their networks. 

Surging demand, driven by people trying to keep their homes warm and cold weather knocking some power stations offline, has pushed Texas' system beyond the limits.

Dan Woodfin, a senior director of system operations at ERCOT, has defended preparations made by grid operators and described the demand on the system as record-setting. 

'This weather event, it's really unprecedented. We all living here know that,' he said. 

'This event was well beyond the design parameters for a typical, or even an extreme, Texas winter that you would normally plan for. And so that is really the result that we're seeing.' 

He said limited supplies of natural gas and frozen instruments at power plants are partly to blame for the blackouts.      

A map from poweroutage.us showed that nearly 5 million people were without power in Texas, and several hundred thousand in Louisiana and Oregon