After decades of decline, one major u.s. industry is making a comeback.

Electric utilities are rebounding, in large part due to one factor – the shale gas boom.

Abundant shale gas means cheap fuel costs for heavy industry. That has caused a number of industries to expand or even re-shore their business in the United States.

More industrial demand for electricity translates to higher capacity utilization and more business for the electric utilities.

It's rather easy to connect the dots in this puzzle.

Expansion on Multiple Fronts

The petrochemical industry is an example of a sector that has expanded rapidly in the United States since the shale gas boom.

This industry, once left for dead, is now optimistic about its future, despite the collapse in oil prices. (Lower oil makes overseas petrochemical products more competitive.)

Since 2010, the industry has made plans for 215 projects worth $133 billion, according to the American Chemistry Council. And 60% of those projects are from foreign companies.

Some electric utilities can already feel the renaissance of American industry, says The Wall Street Journal.

Entergy (ETR) provides power in Texas, Louisiana, Mississippi, and Arkansas. Much of their coverage is in the heart of the Gulf Coast, the center of the petrochemical rebirth. The company saw industrial demand for electricity jump 5.3% in the latest quarter, compared to figures from a year ago.

Southern Company (SO), which provides power in the Southeast, saw demand from its industrial customers rise by 4.8% in the third quarter.

And American Electric Power (AEP) has noticed demand rising nearly across the board. It told The Wall Street Journal that eight out of the 10 industries it serves had increased electricity demand in the third quarter. AEP owns a wide range of utilities, including many in the highly industrial rust belt.

This is all good news, but there is one rather large question looming over the electric power industry.