Like other countries that have promoted the technology with generous state support, Japan is also struggling with the financial and technical consequences of its rapid solar growth. Solar power here is costly for consumers because of high state-mandated prices, and handling the fluctuating output of thousands of mostly small solar producers is tricky for utilities. Necessary improvements in the infrastructure have not kept pace, experts say.

“The homework wasn’t done,” said Nobuo Tanaka, former executive director of the International Energy Agency. Utilities, he said, need to install more hardware — transmission cables, substations and the like — and develop new kinds of expertise to avoid disruptions. “To make renewables work in reality, they have to be properly connected to the power system.”

The problem is especially acute in Kyushu, where relatively plentiful sunshine and low land prices have attracted a disproportionate share of solar development.

Installed solar capacity roughly doubled in the two years from mid-2012, when a law took effect requiring utilities to buy renewable energy from outside producers at rates far above market prices. By last summer it stood at 3.4 gigawatts, about equal to the output of three modern nuclear reactors — at least during those hours when the sun was shining at full strength.

More challenging for electric-company planners is what is in the pipeline. An additional 8.4 gigawatts’ worth of projects, including Mr. Akagi’s on Ukujima, have received government approval but are in limbo after Kyushu Electric’s edict. That is more power than the region consumes on some low-demand days — and far too much for Kyushu Electric’s grid to handle without the risk of failures, the utility argues.

“If we accept everybody’s electricity, the system will become unmanageable,” said Shinichi Futami, an official at the utility. It is laying new transmission cables as fast as it can, he added, but has been stymied by the slow, expensive task of securing land rights.