The blaze was reported at 5:18 a.m. at a home in the 1100 block of St. Francis Drive.
Firefighters responded and found smoke coming from the second story of the home. They extinguished the fire by 5:45 a.m., Fire Inspector George Laing said.
A resident of the home was evaluated for smoke inhalation, Laing said.
Laing said the cause of the blaze appears to be failure of an illegal power bypass and possible power theft.
A man living at the home was detained for questioning by Concord police, Laing said.
Concord police, Contra Costa County fire inspectors and PG&E crews were continuing to investigate the fire this morning, he said.