Electricity prices are heavily regulated. The largest increase I can find from a short search is around 20% for some customers in New Jersey. The average year over year increase is closer to 6%

Unexpectedly high electricity bills are almost always from actual usage. Unexpectedly high winter electricity bills are usually from resistive electric heating in one way or another.

You didn’t mention their normal December bill in this exact house, which is an important piece of information.