Kyushu Electric Power Co.'s recent acknowledgment that it had restricted third-party solar power supplies for two days underscores Japan's need to boost its transmission capacity between regions.

The restriction marked the first time a utility in the country had curbed use of renewable energy to avoid a sudden blackout. It also came just a week after excess renewable supplies were transferred between two of Japan's electricity regions for the first time since a major shake-up of its power sector, according to the nation's grid monitor.

The transfer of excess solar power supplies from Kyushu to Honshu earlier this month had signaled increased flexibility in the country's previously regionalized electricity grid as the market opens up to competition.