There’s a disconnect between when people want to use electricity and when solar tends to produce it. Most often, people use power during the evening or the early morning, when the Sun isn’t yet up.

“There is a mismatch between solar irradiation arriving on Earth and the time when you actually need the energy,” Carsten Streb, a researcher at Elm University, told Ars. “Typically, the energy demand—at least in Germany—is highest in the morning and the evening. Everyone switches on their appliances. But obviously, irradiation is strongest at mid-day.”

One option to handle this mismatch is to use solar power to produce hydrogen, which can then be used at a later time. But keeping the hydrogen around for later use can be a challenge. As Streb told Ars, “One of the big problems we’re seeing with hydrogen is the storage.”