Japan's traditional regional utilities, already defending their territory against nimbler new entrants, face a fresh threat: One of their own is now stealing customers thanks to cheaper nuclear-powered electricity rates.

Kansai Electric Power Co. is one of a few utilities to restart its reactors, which will allow it to offer cheaper rates as it continues its expansion outside of its traditional region, General Manager Michiya Sakata said Friday after releasing earnings.

Seven-Eleven Japan Co. said Thursday that over 3,000 of its stores will switch contracts to Kansai Electric from utilities including Tokyo Electric Power Company Holdings Inc. and Shikoku Electric Power Co. due in part to cheaper rates and a cleaner mix.