ESP32 Deep Sleep Modes and Wake Sources: A Power Optimization Guide

The Power Problem in Battery IoT

An ESP32 running at full CPU speed with Wi-Fi active consumes ~240mA. A 3000mAh lithium-ion battery at this rate lasts roughly 12 hours. For a solar-powered soil moisture sensor or a wildlife camera trap that must run for months unattended, this is completely impractical. Deep Sleep drops current consumption to 10-15μA, extending battery life by up to 1000×.

Understanding ESP32 Power States

| Mode | CPU | Wi-Fi/BLE | RTC | Current | | :--- | :--- | :--- | :--- | :--- | | Active | On | On | On | ~240 mA | | Modem Sleep | On | Off | On | ~15 mA | | Light Sleep | Paused | Off | On | ~0.8 mA | | Deep Sleep | Off | Off | On | ~10–15 μA |

Entering Deep Sleep with Timer Wake

This is the most common pattern: take a reading, transmit it, then sleep for a fixed interval.

#include <esp_sleep.h>

#define SLEEP_DURATION_SECONDS 300  // Wake every 5 minutes

void setup() {
  // Do sensor reads and upload here...
  readAndUploadSensorData();

  // Configure timer wake source
  esp_sleep_enable_timer_wakeup(SLEEP_DURATION_SECONDS * 1000000ULL);

  // Enter deep sleep immediately
  esp_deep_sleep_start();
}

void loop() {
  // Never runs — device wakes as a fresh boot after sleep
}

Persisting State Across Sleep Cycles with RTC Memory

Normal SRAM is cleared during deep sleep. RTC memory (8KB on ESP32) survives sleep cycles:

RTC_DATA_ATTR int bootCount = 0;  // Persists across resets
RTC_DATA_ATTR float lastTemperature = 0.0f;

void setup() {
  bootCount++;
  Serial.printf("Boot number: %d\n", bootCount);
  // bootCount persists even after deep sleep
}

GPIO Wake Source (External Interrupt)

For event-driven devices (door sensors, motion detectors), wake on pin state change instead of a timer:

// Wake when GPIO 33 goes HIGH (e.g. PIR motion sensor output)
esp_sleep_enable_ext0_wakeup(GPIO_NUM_33, 1);
esp_deep_sleep_start();

Battery Life Estimation

For a sensor that wakes every 5 minutes, spends 3 seconds active (Wi-Fi connect + transmit), then deep sleeps:

• Active: 3s × 240mA = 0.2mAh
• Sleep: 297s × 0.015mA = 0.0012mAh
• Per cycle: ~0.2mAh
• Daily: 0.2 × 288 cycles = 57.6mAh
• 3000mAh battery → ~52 days runtime

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