NEW YORK – A plunge in the price of natural gas has made it cheaper for utilities to produce electricity. But the savings aren’t translating to lower rates for customers. Instead, U.S. electricity prices are going up.

Electricity prices are forecast to rise slightly this summer. But any increase is noteworthy because natural gas, which is used to produce nearly a third of the country’s power, is 43 percent cheaper than a year ago. A long-term downward trend in power prices could be starting to reverse, analysts say.

“It’s caused us to scratch our heads,” said Tyler Hodge, an analyst at the U.S. Energy Department who studies electricity prices.

The recent heat wave that gripped much of the country increased demand for power as families cranked up their air conditioners. And that may boost some June utility bills. But the nationwide rise in electricity prices is attributable to other factors, analysts say:

• In many states, retail electricity rates are set by regulators. As a result, lower power costs haven’t yet made their way to customers.

• Utilities often lock in their costs for natural gas and other fuels years in advance. That helps protect customers when fuel prices spike, but it prevents customers from reaping the benefits of a price drop.

• The cost of actually delivering electricity, which accounts for 40 percent of a customer’s bill on average, has been rising fast as utilities build transmission lines and install new equipment.

The average U.S. residential electricity price is expected to be 12.4 cents per kilowatt hour for the June-to-August period, up 2.4 percent from the same time last year. For the full year, electricity prices are expected to rise 2 percent.

In a typical summer month, that would mean an extra $3 on a residential bill, which includes the cost of generating the power and delivering it to a home, plus local taxes and fees.