Traditional transmission and distribution utilities will have to deal with distributed solar power, and it won't be a pretty fight, according to David Crane, president and chief executive of NRG Energy NRG +0.19%, a large independent power producer.

Utilities "do realize that distributed solar is a mortal threat to their business," said Mr. Crane, speaking at The Wall Street Journal's ECO:nomics conference on Thursday in Santa Barbara, Calif.

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