The number arrived without ceremony, in a quarterly auction result: four-hour grid storage cleared below the threshold at which solar-plus-batteries undercuts new gas peaker plants on pure economics, no subsidy required. Energy modellers have treated that crossing as the hinge of the transition for a decade; it is now a market price.
The descent came from unglamorous compounding — cell chemistry iterations, manufacturing scale, financing costs falling as bankers learned the asset class. Duration is stretching in parallel: eight-hour systems dominate next year's auction pipelines, and the first hundred-hour iron-air installations are under construction on three continents.
Grid planners describe the consequence in operational terms: solar's midday glut becomes evening supply, curtailment turns into revenue, and the marginal new megawatt in most of the sunbelt is now firm renewable power. The debate about whether renewables can carry a grid, one system operator remarked, has quietly become a scheduling question.