Prices are rising because customers want an alternative to utilities that use coal, which causes more pollution, Ellerbrook told investors at an annual meeting.
Ten years ago, the cost of natural gas was about $2.50 a dekatherm; it is projected to hit $11.30 next month.
Natural gas prices also will rise because lawmakers are expected to require utilities to cut emissions. That means companies will have to buy equipment to keep pollutants from being emitted from their plants, and the costs will be passed to the consumer.
The Evansville-based utility serves most of central Indiana outside of Indianapolis.