How battery life is estimated

A deep-sleeping IoT node spends most of its time asleep and wakes briefly to measure and transmit. The calculator averages the two phases over one wake cycle — I_avg = (I_active·t_active + I_sleep·t_sleep) / t_cycle — then divides the usable battery capacity by the daily consumption. Usable capacity defaults to 80%, because you never get the label value: voltage cutoff, temperature and self-discharge all eat into it.

Typical numbers: a bare ESP8266 draws ~70–120 mA while WiFi transmits and ~20 µA in deep sleep; an ESP32 ~100–240 mA active and ~10 µA in deep sleep. Beware development boards: the USB-serial chip and onboard regulator on a NodeMCU or DevKit can leak 1–10 mA even in deep sleep, cutting battery life by orders of magnitude — for battery projects use a bare module or cut the offending traces. Related: ESP & Arduino Pinout Reference.