Safe indoor temperature limits shape overheating resilience and cooling demand in California homes

As climate change intensifies, the frequency and severity of extreme heat events are increasing, placing growing stress on building environments and energy systems. Using a physics-based residential building-stock framework, we conducted over 265,000 whole-building simulations (representing over 50,000 California homes across five climate scenarios) to quantify indoor overheating severity and the resulting increases in cooling demand. Indoor heat exposure was evaluated using the indoor overheating degree (IOD), which captures both the duration and magnitude of temperature exceedances above selected comfort limits. Results show that by the 2080s, up to ~ 55% of homes without cooling may exceed safe indoor limits, driving a 20–40% rise in statewide peak cooling electricity loads and potentially surpassing grid adequacy margins. Overheating risk moves from inland where AC is more common to coastal regions where it is not, increasing overall indoor heat exposure while narrowing regional disparities. By linking thermal limits with AC adoption and grid impacts, we quantify where overheating risks and cooling burdens intensify under current and future climates, providing evidence to support thermally safe and grid-aware housing policies.