The world’s biggest-ever solar project—a $200 billion venture in Saudi Arabia—comes with a “batteries included” sticker that signals a major shift for the industry.

SoftBank Group Corp. partnered with the oil-rich Saudis this week to plan massive networks of photovoltaic panels across the sun-drenched desert kingdom. The project is 100 times larger than any other proposed in the world, and features plans to store electricity for use when then sun isn’t shining with the biggest utility-scale battery ever made.

The...