New England power plants burned more oil for electricity on a single day during last month’s deep freeze than they have in four years, underscoring the gap between Northeastern states’ clean energy targets and the current resource mix in the region.

Oil resources supplied 29 percent of a six-state region’s power on Dec. 24 as temperatures hovered in the teens, natural gas supplies tightened and some generators failed to perform as expected. The amount of electricity generated by oil that day was higher than it had been since a weekslong polar vortex hit New England in January 2018, according to an E&E News review of annual reports from the regional grid operator on fuel use.

New England and New York are the only parts of the country that rely extensively on oil resources for backup power when other electricity supplies are expensive or in short supply. In both regions, oil is used sparingly throughout the year, having accounted for 0.2 percent of the total electric load in New England in 2021, according to ISO New England, the area’s nonprofit grid operator.

“It’s been years that this back and forth switching between fossil fuels has been going on, and it’s not improving,” Amy Boyd, vice president of climate and clean energy policy at Acadia Center, a New England-based environmental advocacy group, said in an email. “We need to instead come to a better solution.”