In answering this question, it"s important not only to consider the upfront expenditure, but the total lifetime cost of a system. Gravity batteries are mechanical contraptions, and as such, they can break. Maybe a cable snaps, or a gearbox jams, or a patch of rust appears. These issues are problematic, but they"re not fatal – individual components can be replaced with relative ease. This "repairability" means gravity batteries can last as long as 50 years, says Asmae Berrada, an energy storage specialist at the International University of Rabat in Morocco. (Read about the big unanswered question surrounding lithium batteries.)