Power Monitoring

A device based on PZEM-004T that monitors power, current, voltage, frequency, etc.

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Yet another device that monitors power, current, voltage, frequency, power factor, and energy. The live data is available on a screen or mobile-friendly web app.


  • Shows predefined metrics on the screen
  • Sends values to the MQTT broker
  • Auto-resets energy metric at the beginning of the month
  • Shows the previous month's energy value on the UI
  • Supports Home Assistant auto-discovery
  • Web app
    • Simple web app that updates metrics via WebSockets
    • Shows warning when value not in a range
    • Stores ranges in EEPROM and allows configuring them on the UI
    • Stores MQTT configuration in EEPROM and allows configuring it on the UI
    • Progressive web app (PWAs), so the user can install it


3mf - 15.69 kB - 01/08/2022 at 10:38



3mf - 6.79 kB - 01/08/2022 at 10:38


  • 1 × PZEM-004T v3.0
  • 1 × WT32-ETH01
  • 1 × 0.96" OLED display SSD1308 128x64
  • 1 × DIN Rail Box 88x37x59mm

  • Integration with Home Assistant, bug fixes

    strange.rand11/23/2022 at 21:38 0 comments

    Just a quick update. The Power Monitor works almost a year without severe issues. As I expected, the OLED started to fade, so switching the display off base on a light sensor (or whatever) is essential (but won't be implemented in this hardware version).

    As I'm playing with Home Assistant, I decided to add auto-discovery to the device to simplify integration. New options on the settings page:

    And this is how HA displays it:

    Additionally, many bug fixes were added recently to improve stability and code quality. BTW, the charts were entirely removed.

    That's all for now.

  • Time sync, reset energy (manual), and charts

    strange.rand01/19/2022 at 07:24 0 comments

    Here is a short (hopefully) log about the recent updates.

    Time sync (NTP)

    During searching for an optimal NTP library for ESP32, I've found out that the time syncing functionality can be added simply by including <time.h> header.

    First, we need to configure time. It can be done in two ways:

    • using configTime(<GMT offset>, <DST offset>, <NTP server>)
    • using configTzTime(<timezone>, <NTP server>)

    The second option is a great way to automatically handle switching to daylight saving time. The required timezones can be found here: zones.csv. Then we can use getLocalTime method to retrieve a local time. Time syncing will be executed automatically every hour (by default).

    Reset energy

    It can be done using a new menu:


    This is the most controversial functionality (or the most useless one). I wanted to store and show power and voltage for the last 12 hours (it's enough for basic stuff). Let's do quick math:
    12 × 60 × 60 = 43200 datapoints
    (4 + 4 + 4) × 43200 = 12 × 43200 = 518400 bytes
    It is a lot, so I decided to aggregate data per minute, that's reduces needed memory to 8640 bytes. After additional thinking, the decision to store min and max was made, so the final structure looks like this:

    struct ChartData
        uint32_t date;
        float minVoltage;
        float maxVoltage;
        float minPower;
        float maxPower;

    Two circular buffers are used to store the values. One is processed and cleared every minute, and one is for "permanent" storage.

    CircularBuffer<ChartData, 720> chartBuffer;
    CircularBuffer<TempChartData, 60> tempChartBuffer;

    ArduinoJson library is not used to build the final JSON response to additionally reduce memory consumption.

    Selecting a proper chart library was hard. I started from Google Charts, then tried ChartJS, one or two other libs, and then finally uPlot. It is small, fast, and has enough features to display what I want though, it also has a big disadvantage – pure documentation.

    Even considering that the chart supports zooming and panning, using it on the smartphone isn't comfortable. But it was an interesting process to build it. One additional thing I like to try here – streaming data directly from the circular buffer without an intermediate char array (it'll drastically reduce memory consumption).

    Next steps

    • An automatic reset of the energy counter
    • Refactoring
    • Bugfixes

  • Settings and WebApp

    strange.rand01/07/2022 at 12:08 1 comment

    Many things were added/improved during the last few days:

    • Settings added (EEPROM storage is used)
    • Improved serving of static files
    • Introduced an automated build process for WebApp (using Webpack)
      • build-in dev server with hot reload
      • production build
      • minification of files
      • file compression (gzip)
      • inline SVG icons (to serve fewer files)
    • Restructured WebApp
    • Added caching
    • Added notification about an update available (PWA specific, more details later)
    • Possibility to install the app is added
    • Improved UI/UX

    Read more »

  • Firmware

    strange.rand01/02/2022 at 07:55 0 comments

    How to upload firmware

    WT32-ETH01 requires a standalone USB to UART adapter for firmware uploading. Wiring:

    Pins IO0 and EN should be connected to GND only to start uploading firmware and then disconnected to allow the normal boot.

    To configure wired ethernet, we have to add a few defines. It can be done in the code (in that case, the defines should go before "#include <ETH.h>") or in the platformio.ini using build_flags.

    platform = espressif32
    board = esp32dev
    framework = arduino
    monitor_speed = 115200
    build_flags = 
        -D ETH_PHY_ADDR=1
        -D ETH_PHY_POWER=16

    Code structure

    I like keeping different things in different files instead of having a bulky "main.cpp". Let's see what functionality can be separated:

    • Network: connect, configure,  reconnect, etc.
    • Pzem: get data, reset energy counter (in the future)
    • Display: show required information on the screen
    • Webserver: configuration, all handlers for URLs, update clients via WebSockets
    • MQTT: connect, reconnect, configuration, send data
    • Module: some base, reusable functionality

    So the current structure is:

    For example, Mqtt.h describes everything related to MQTT. If you want to use a variable that is defined in main.cpp, you have to define it as an external in the Mqtt.h (e.g., extern EventGroupHandle_t eg).


    As you probably know, the Arduino core for ESP32 uses FreeRTOS, which gives us a lot of possibilities. Let's analyze our needs and check what tasks we have:

    • Get data from the PZEM module
    • Update display
    • Send updates to web clients
    • Send data to MQTT broker

    The first task is repeatable, and in my case, it should be executed once per second. The next three tasks should be executed only if we have new data to process, so they are event-based.

    First of all, we have to define a few things:

    EventGroupHandle_t eg;
    QueueHandle_t qMqtt;
    TimerHandle_t tRequestData;
    SemaphoreHandle_t sema_PZEM;

    Then in the setup function:

    eg = xEventGroupCreate();
    qMqtt = xQueueCreate(4, sizeof(MqttMessage));
    sema_PZEM = xSemaphoreCreateMutex();
    xTaskCreatePinnedToCore(taskRetrieveData, "RetrieveData", TaskStack10K, NULL, Priority3, NULL, Core1);
    xTaskCreatePinnedToCore(taskUpdateDisplay, "UpdateDisplay", TaskStack15K, NULL, Priority3, NULL, Core1);
    xTaskCreatePinnedToCore(taskUpdateWebClients, "UpdateWebClients", TaskStack10K, NULL, Priority3, NULL, Core1);
    xTaskCreatePinnedToCore(taskSendMqttMessages, "tMqtt", TaskStack10K, NULL, Priority2, NULL, Core1);
    tRequestData = xTimerCreate("RequestData", pdMS_TO_TICKS(Cfg::requestDataInterval), pdTRUE, (void *)0, reinterpret_cast<TimerCallbackFunction_t>(requestData));

    The request data method is just one line of code

    void requestData()
      xEventGroupSetBits(eg, EVENT_RETRIEVE_DATA);

    It fires an event using the event group (eg) defined above. Theoretically, we can do real work here, but without blocking, so better to have a separate task.

    EVENT_RETRIEVE_DATA is defined in the Pzem.h like this

    #define EVENT_RETRIEVE_DATA (1 << 1)

    Below you can see simplified code for retrieving the data. It waits for the event, takes a semaphore and retrieves the data, then gives the semaphore back, fires events to update display and web clients, and finally, composes MQTT message and pushes it to a queue. All other tasks are based on the same principles.

    void taskRetrieveData(void *pvParameters)
        for (;;)
            xEventGroupWaitBits(eg, EVENT_RETRIEVE_DATA, pdTRUE, pdTRUE, portMAX_DELAY);
            if (xSemaphoreTake(sema_PZEM, TICKS_TO_WAIT12) == pdTRUE)
                // read data from PZEM here
                xEventGroupSetBits(eg, EVENT_UPDATE_DISPLAY | EVENT_UPDATE_WEB_CLIENTS);
                MqttMessage msg = composeMessage(data);
                if (xQueueSendToBack(qMqtt, &msg, 10) != pdPASS)
                    debugPrint("Failed to add to the mqtt queue");

     As a result

    • We have a neat and clear code structure
    • Software timer is used to initiate a repeatable task...
    Read more »

  • Heating

    strange.rand01/01/2022 at 17:02 0 comments

    A few thermal photos of the module and its components:

    Front view

    I didn't expect that this board could produce so much heat.

    PZEM-004T v3.0

    The hottest part here is a resistor.

    Overall, it is okay, but better to use something more thermally resistant than PLA for 3D printed parts.

  • Hardware

    strange.rand12/31/2021 at 14:59 0 comments

    The module consists of 3 main components:

    • PZEM-004T v3.0 – an upgraded version of energy meter
    • WT32-ETH01 – well knows ESP32 but with wired ethernet connection
    • OLED display (SSD1308 128x64)

    A few words about the component choice. WT32-ETH01 was chosen because I don't want to rely on WiFi and need a wired ethernet connection. There are a few alternative bords, but this one is relatively small and cheap. Instead of an OLED display, I'd rather use a color TFT (IPS), but they require an SPI bus that we don't have on WT32-ETH01.

    The module should be mounted on a DIN rail, so compatible housing was ordered from Aliexpress.

    PZEM-004T PCB does not fit into the bottom part of the housing; it is a bit too wide in the middle, where the latch is, though it can be easily fixed with a file.

    To secure everything in place, I modeled and 3D printed a custom "holder" that you can see in the photo below:

    It has holes to screw PZEM-004T PCB, and it also stops WT32-ETH01 from moving around (especially when connecting ethernet cable).

    To secure the screen, one more part was 3D printed and the screen was hot-glued to it. Then the whole sandwich was hot-glued to the housing itself. All required holes in the DIN rail housing were made by drill and a file (they are way from perfect).

    The final part is to wire everything up. I use thin, manually crimped silicon wires for this (also from Aliexpress). Pay attention that RXD pin is not usable on the board, so IO14 was used instead.

    Unfortunately, I didn't find a neat way of connecting power to the module, so I just left wires.

    A fun part here. The DIN rail housing is a bit non-standard, so it was impossible to feet it properly in the electric shield. Luckily, this can be fixed with a bit of sanding.

    One more thing was planned but forgotten in the process – adding a button or photoresistor, so the screen can be turned off:

    • after some period (in the case of using a button)
    • when there is on light (in the case of using a photoresistor)

    It would significantly prolong the life of the OLED screen.

    The next log will be about software, as it is ready to some degree.

View all 6 project logs

Enjoy this project?



Ed wrote 10/27/2023 at 08:01 point

Thank you so much. I will give it a try and will give you update!!!

  Are you sure? yes | no

junwoo091400 wrote 01/16/2022 at 13:04 point

This is a fascinating project!

  Are you sure? yes | no

strange.rand wrote 01/16/2022 at 13:20 point

Thank you!

  Are you sure? yes | no

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