As tropical storm Barry roared ashore in Louisiana last month, Entergy Corp. monitored the electric grid using airwaves that have long been reserved for utilities and first responders.

Those secure, wireless signals helped the company manage outages, even while winds of up to 65 miles (105 kilometers) per hour lashed the region. But as the world becomes increasingly wireless, U.S. regulators plan to open more of those airwaves to tech companies.

The move pits two of the most powerful U.S. industries against each other. Tech giants including Apple Inc. and Facebook Inc. contend the airwaves, in the 6 gigahertz range,...