The ESP32 has 8192 bytes of real Time Clock (RTC) RAM that can be used to store variables during periods of deep sleep. Using a simple compiler directive called RTC_DATA_ATTR enables data to be retained during sleep. This provides the ability to save; for example sensor readings, during successive readings and then exporting then in bulk. Up to 800 integer or floating point readings can be stored.
Code Examples: https://github.com/G6EJD/ESP32_RTC_RAM