Japan's solar boom is beginning to falter.

Until recently, the resource-poor nation has been one of the leading markets for photovoltaic (PV) units, helping to prop up an industry hurt by falling prices for the technology and policy changes. But four years after the introduction of generous incentives to promote clean energy in the wake of the March 2011 Fukushima nuclear disaster, data show the boom is losing steam.

The slowdown — after several years of rapid growth — threatens to undermine the government's push to find a clean alternative to nuclear power and dims what has been a bright spot for the global photovoltaic industry.