“We are providing them a solution to reduce their energy cost and demand cost,” said Lyndon Rive, SolarCity’s chief executive, adding that the systems would provide backup power and a working solar array during blackouts. Although he said he did not see the systems as a step toward independence from the grid, storage would be important to maintaining grid stability as more customers adopt solar.

The product grew from a $1.8 million grant in 2010 from the California Public Utilities Commission to study the possibilities of storing electricity from rooftop solar arrays in batteries. The company has also signed up about 300 of its residential customers for a pilot program using battery packs from Tesla, whose chief executive, Elon Musk, is the chairman of SolarCity and Mr. Rive’s cousin.

The company will begin offering its storage systems in parts of California, Connecticut and Massachusetts. Although the combination of solar with storage could prove potent, said Sam Jaffe, a senior research analyst at Navigant Research, the market for it is by no means assured.

“It’s not a no-brainer — you still have to have the exact right combination of rates and demand charges and the cost of that equipment,” he said. He added, though, that if the systems are configured to take advantage of a 30 percent federal tax credit, the economics would have broader appeal.

The SolarCity systems, which are designed to be eligible for the credit, will be made available to new solar customers signing 10-year service agreements and are made to store about a third of the energy the solar array can produce. Mr. Rive said the company would guarantee the demand charge reductions, which he expects to run around 20 percent.