Panasonic has had a rough 15 years. Founded in 1918, the Japan Inc. icon long stood tall with Canon, Hitachi and Sony. By the time the 2000s arrived, its televisions, radios and bicycles seemed obsolete. Panasonic's Osaka-based leadership, more out of desperation than prescience, shifted to housing and batteries.

Then came a phone call from Elon Musk. When the Tesla Motors guru began building his $5 billion lithium-ion gigafactory in Nevada in 2014, he turned to Panasonic and its decades of research and development on next-generation battery technology. Now, Tesla is deepening its collaboration with Panasonic to dream up a solar-energy revolution that could enliven not just these two companies, but Japan's economy.

For Musk, tapping Panasonic is a shrewd way to fuel his SolarCity game plan. He's both chief executive of Tesla and chairman of SolarCity and sought to combine them to offer the world one-stop shopping for vehicles and the renewables on which they'll run. Shareholders are less enamored with a marriage they see as fraught with corporate-governance risks. Collaborating with Panasonic may help Musk close the deal and placate analysts who see him more as a cash-burning charlatan than visionary changing the concept of energy forever.