What deep sleep does
Deep sleep turns off most ESP32 blocks and keeps only selected wake-up sources alive. This can dramatically reduce average current.
Wake time matters
Battery life depends on both sleep current and wake duration. A short wake interval with long WiFi connection time can still drain a battery fast.
Board selection
Development boards may have USB chips, LEDs and regulators that prevent ultra-low sleep current. Bare modules or low-power boards work better.
Practical tips
Minimize wake time, batch sensor readings, reduce WiFi connection time and use a regulator with low quiescent current.