As residential batteries have become more energy dense, cheaper, and smaller, more households are storing their excess solar power. Now, utilities and energy companies in dozens of countries are buying up those electrons, bundling them together, and using them to balance the grid.

Home and commercial solar arrays provide nearly a fifth of Australia’s electricity generation, with panels atop one in every three homes. To extend those panels’ usefulness, owners are increasingly buying home batteries not only to store their power for later use, but to sell electrons to the grid at times of high demand. The arrangement enables grid operators to more effectively manage the mismatch between midday solar generation and real-time consumer demand, a process known as balancing.…