Understanding your temperature derate result
Why battery capacity changes with temperature at all
Battery capacity depends on how fast ions can move through the electrolyte and electrodes, and that movement slows down in the cold and speeds up in the heat, at least until heat starts causing other problems. Cold temperatures reduce the capacity actually available on a given day, even though the battery's long-term rated cycle life stays the same once it warms back up, which matters most for a system sitting through a real seasonal extreme, like the scenario covered in the seasonal cabin battery storage over winter tool.
Why cold weather affects lead acid more than lithium
Lead acid batteries rely on a liquid or gel electrolyte that thickens noticeably in cold weather, which is why lead acid capacity often falls further than lithium chemistries at the same low temperature, an electrolyte-viscosity effect NREL's energy storage research has characterized across chemistries at a range of operating temperatures. This is one of several reasons covered in the best chemistry for extreme climate comparison for choosing a chemistry suited to a specific region.
Why heat is a longer-term problem, not just a short-term one
Capacity often holds up reasonably well at moderately hot temperatures in the short term, but sustained heat exposure accelerates the same chemical aging processes covered in the capacity over time calculator, permanently shortening the battery's life even if today's output looks fine. Once the chemistry is settled, size the bank. The battery charge time from solar calculator in our Battery & Storage section can help you work out the capacity your daily load needs.
What to check next
If your operating temperature runs to either extreme regularly, the thermal stability comparison covers the safety side of hot-weather exposure, and the cold and hot weather capacity tools linked above go into more region-specific detail. This same temperature-dependent performance question factors into how the DOE's Office of Electricity evaluates storage system reliability across different climate zones.