Now, though, with so many solar panels on so many rooftops, critics say Germany has too much of a good thing. Even at a time of record oil prices, solar is encountering resistance from conservative lawmakers who want to pare back its generous state incentives. They say it is growing at an unhealthy pace, threatening to burden consumers with too many costs in the form of higher electricity bills.

Solar-energy entrepreneurs warn that reducing these incentives would deprive Germany of its pole position in an industry of tomorrow. They liken Germany to the United States and Japan, which were both once solar stars but faded as their subsidies became less attractive.

The debate over solar energy subsidies is a test of how an environmentally-minded country can move from nurturing a promising alternative energy to creating a mass-market industry that can compete, on its own footing, with conventional energy sources. It is a tricky transition, even with a sympathetic population.