The company’s first solar apartment went on sale last June, in the city of Wako in Saitama Prefecture, near Tokyo, and sold almost instantly, according to the company. Takara Leben — like its rivals Sankei Building and Daikyo, which have developed similar projects recently — had conceived the solar-apartment idea before the earthquake. But the disaster “gave a big boost” to sales of the 112 units, Mr. Kitagawa said. The price tags were ¥30 million to ¥38 million, or roughly $350,000 to $470,000.

The company was so happy with the results that it now has five projects in the pipeline, two to be completed this spring.

Sankei Building, another midsize developer, had also set out to build a solar apartment in 2010 in Musashino, a popular suburban residential district of Tokyo. Last June, the company announced its sales to great acclaim. “They sold out quite instantaneously,” said Yukari Sasaki, the managing officer who heads the residential development department at Sankei in Tokyo. “People’s awareness for natural energy and disaster readiness has been greatly enhanced” after the earthquake.

Each apartment’s solar system comes with control panels and a display that compares energy generation and use on a month-to-month and year-to-year basis. The apartments also have batteries that kick in when grid-supplied energy is cut in emergencies. Sankei Building has three new solar apartment buildings in the works.

There is another reason midsize developers favor solar apartments. They tend to build on the outskirts of Tokyo, while their bigger rivals, like Mitsui Fudosan, Mitsubishi Estate and Sumitomo Realty, have an edge in developing inner-city high rises in prime locations. (Procuring land in a good location is a matter of business clout.) But high-rises are generally unsuitable for solar apartments, because they require expansive roof space relative to the number of units.