The arctic blast that’s turning the northern half of the U.S. into a giant icebox this week has been good news for oil and coal.

Plants were using the most fuel oil in three years Dec. 28 to produce the electricity that’s powering heaters across New England. In the PJM market, which stretches from Illinois to the District of Columbia, coal has once again surged past natural gas to become the biggest fuel for power generation. Oil demand there jumped...