ESP32 vs STM32: Which MCU for Your IoT Project?

📅 October 2025 ⏳ 3 min read FSS Engineering Team

MCU

ESP32-S3 and STM32L5 — the two MCU families at the core of FSS IoT products

After building hundreds of IoT devices on both platforms, we have clear opinions on when to use each. The short answer: ESP32 for WiFi/BLE consumer products, STM32 for industrial, ultra-low power, or safety-critical applications.

Connectivity: ESP32 Wins

ESP32’s integrated WiFi (802.11 b/g/n) + Bluetooth 4.2/5.0 is its killer feature. A single $3 module gives you both protocols with no external components. The ESP32-S3 adds USB OTG and a vector processing unit for local ML inference.

Power: STM32 Wins

STM32L4 achieves sub-1μA standby with full RTC. ESP32 deep sleep draws 10–150μA. For coin-cell sensors targeting 5+ year battery life, this difference is decisive.

// Real sleep current measurements

STM32L476RG Stop 2 (RTC on):  ~0.4 μA
ESP32-WROOM Deep sleep:       ~10 μA
ESP32-S3 Deep sleep:          ~8 μA
nRF52840 System OFF:          ~0.4 μA

Security Features

STM32U5/L5 include hardware TrustZone (ARM Cortex-M33) and AES/SHA/RSA accelerators. ESP32-S3 has hardware crypto acceleration and eFuse-based secure boot — sufficient for most commercial IoT products.

💡 FSS default choice

For new connected product designs: ESP32-S3 if WiFi or BLE is needed and power is not critical. STM32U5 if <1μA sleep is required. nRF52840 for BLE-only with ultra-low power.

Talk to our hardware team about the right MCU for your product.

Development Ecosystem and Toolchain

ESP32’s ESP-IDF (IoT Development Framework) is CMake-based, well-documented, and has an enormous active community. Arduino framework support means you can prototype quickly and migrate to ESP-IDF for production. The ecosystem of libraries — from MQTT clients to sensor drivers — is unmatched in the embedded world at this price point.

STM32CubeIDE generates peripheral initialisation code through a graphical configurator (CubeMX), which reduces boilerplate but can produce overly verbose HAL code. The underlying hardware abstraction is clean once you understand it. For teams coming from an embedded background, STM32 feels more “proper” — direct register access is straightforward, and the peripheral documentation is thorough.

Production Cost and Supply Chain

At volume, cost matters. An ESP32-S3 module (ESP32-S3-WROOM-1) costs around $2.50 at 1,000 units. An STM32U5 in a comparable package costs $4–7 depending on flash size. For a consumer product shipping 50,000 units per year, that difference is significant. However, STM32 is available from multiple authorised distributors with strong supply chain guarantees — important after the 2021–2023 chip shortage that hit ESP32 supply harder than STM32.

Certification Implications

The ESP32-WROOM and ESP32-C3-MINI modules come pre-certified for FCC and CE. Using a certified module means your host device only needs Class B verification for emissions — a significant cost and time saving. STM32 by itself has no radio, so certification cost depends entirely on the external radio module you choose. This is actually an advantage: you pick exactly the module with the certification path that suits your timeline and budget.

For most new IoT product designs starting in 2026, our recommendation remains ESP32-S3 for connected consumer products and STM32U5 for industrial, regulated, or ultra-low-power applications. The choice should be driven by your power budget, security requirements, and the radio protocols your product needs — not by familiarity alone.