'Massive failure': Why are millions of people in Texas still without power?

Power outages across Texas have left millions of people in the dark and bitter cold this week amid an unprecedented winter storm that buried the state in snow and ice and brought single-degree temperatures.

Extreme energy demand and overloaded frozen utility plants are among the factors that led to the power outages, experts said.

"No matter which way you cut it, this is a massive failure for a grid and a state that holds up energy and electricity as a shining example," said Varun Rai, director of the Energy Institute at the University of Texas-Austin.

Nearly 4.5 million customers went without electricity Tuesday, and by Wednesday over 3.3 million Texans still didn't have the lights turned on, according to poweroutage.us.