Store ESP32 settings with Preferences (NVS): limits and a safe pattern

Preferences is the Arduino wrapper around ESP-IDF's NVS (non-volatile storage). Data lives in the nvs flash partition and survives restarts, deep sleep and power loss.

Limits that bite

Keys longer than 15 characters fail; this is a frequent reason for "settings are not saved".

Pattern

#include <Preferences.h>

Preferences prefs;

struct Config {
  String ssid;
  uint32_t intervalSec;
};

Config loadConfig() {
  Config c;
  prefs.begin("app", true);                       // read-only: writes would fail
  c.ssid = prefs.getString("wifi_ssid", "");
  c.intervalSec = prefs.getUInt("interval_s", 300);  // default if missing
  prefs.end();
  return c;
}

void saveInterval(uint32_t seconds) {
  prefs.begin("app", false);                      // read-write
  if (prefs.getUInt("interval_s", 0) != seconds) { // don't rewrite unchanged values
    prefs.putUInt("interval_s", seconds);
  }
  prefs.end();
}

Tips

• Always pass a default to get*() so a fresh or erased device boots with sane values.
• Skip unchanged writes. NVS spreads writes across the partition, but flash still wears; don't write a value on every loop iteration.
• Version your schema: store a cfg_ver key and migrate old layouts on boot.
• Credentials: NVS is not encrypted unless you enable NVS encryption (ESP-IDF feature). Anyone with the flash contents can read plain NVS.
• Erasing flash (esptool erase_flash) also erases NVS.