Exceptionally harsh winter weather that surprised Texas authorities was the leading cause of widespread generator outages that left millions of residents without power for days in February, a new report says.

The initial findings from the Electric Reliability Council of Texas (ERCOT), which oversees electric power in the state, amplified the already widely reported thinking of independent analysts that energy facilities and infrastructure in the state were ill-equipped to withstand a prolonged deep freeze. Half of the generation in ERCOT was forced offline during the storm.

A polar vortex that swept deep into the Lower 48 brought frigid air and snow as far south as Houston – a rarity. Average temperatures were about 30 degrees below normal during the Feb. 14-19 deep freeze...