By Mike DeNardo

PHILADELPHIA (CBS) — It’s been an especially warm December. So why are PGW customers seeing an additional charge on their natural gas bills?

The extra PGW charge is called a Weather Normalization Adjustment. It’s a utility’s way of softening the revenue swings that come with extremely warm or extremely cold weather, says Nils Hagen-Frederiksen, Press Secretary for the state Public Utility Commission.

“If a utility is using this type of adjustment and things are unusually warm, as they are right now, consumers will see that additional charge on their bill. But as we’ve seen in past winters, it’s also on the bill as a credit or a partial refund when things are colder than normal.”

PGW is one of only two Pennsylvania utilities using the WNA. The PUC in March holds a hearing on utilities’ strategies to decouple revenue from how much electricity or gas they sell.