FRANKFURT—When Christian Hurtz opened his electricity bill just before New Year, his jaw dropped: it had more than trebled from the rate he signed up for. The 41-year-old software developer from Cologne, Germany, is one of millions of Europeans who have seen their energy costs balloon as providers go out of business because of soaring gas prices or pass them on to customers. Spending more on heating, lighting or running a car is straining many households’ budgets and shaking expectations that a consumer-led economic boom will follow pandemic-era restraint. “At first I thought that was the amount for three months,” said Hurtz, whose bill came from the provider of last resort after his own energy company stopped supplying. “When I realised they wanted it every single month, my jaw dropped. It spoiled my Christmas break a bit,” he told Reuters. In 2020, euro zone households spent an average of $1369 …