The run rate fell further from June's 36.8 percent and was less than half the 70.0 percent rate a year earlier. It also dipped below the 34.2 percent rate for May 1979, the last month for which data was available.
The prolonged radiation crisis at Tokyo Electric Power's Fukushima Daiichi plant has stirred fears about nuclear safety, leaving local governments wary about granting approvals to restart reactors taken offline for maintenance.
The government plans safety tests of Japan's reactors and is reviewing its energy policy as the country confronts the prospect of power shortages that could stretch into 2012.
Only 16 reactors are still operating of the 54 commercial reactors available to Japan's nine Japanese utilities and a non-utility electricity wholesaler, Japan Atomic Power Co, at the time the March 11 earthquake and tsunami that triggered the Fukushima crisis.
By the end of September, another five reactors are scheduled to shut for maintenance, and all of Japan's reactors could be shut by next May if there are no restarts. Read More