Solar Giant Sunrun Sees Vivint Deal as Key to Battery Growth (1)

Sunrun Inc., America’s largest residential-solar company, is about to become even bigger by acquiring one of its top rivals. The deal will also help redefine it as more than just an installer of rooftop panels and batteries.

For Sunrun, buying Vivint Solar Inc. is a way to accelerate a key objective: clustering thousands of batteries in dense areas that the company can tap to provide power to local electrical grids.

That focus shows how rooftop-solar providers are maturing. Installing batteries along with panels gives them a supply of dispatchable electricity that can be sold. As a result, companies that ...