Build an ESP32 energy meter with a live dashboard

Most ESP32 energy-meter tutorials you'll find are built on platforms that have since changed out from under them — deprecated app flows, retired tokens, code that no longer compiles as written. This build has no such dependency. A PZEM-004T does the metering, the ESP32 reports over plain HTTPS/WebSocket, and the dashboard, history, and alerts run in nodrix on your own Cloudflare account — nothing in the path can be discontinued on you.

What you get: live volts, amps, watts, and a lifetime kWh counter on a dashboard you can open from anywhere, a 24-hour load curve, and a Telegram message when something draws more than it should.

Safety first

This project meters mains electricity. The ESP32 side is all low-voltage, but the PZEM's input terminals connect to live line and neutral:

• De-energize the circuit before touching any mains wiring, every time.
• Enclose everything. No exposed screw terminal when powered — a cheap junction box is fine.
• Fuse the voltage tap with a small inline fuse (0.5A is plenty; it only feeds the meter).
• The CT clamp is non-invasive — it clips around one insulated conductor (line or neutral, never both) and touches no copper.

If any of that reads as unfamiliar rather than routine, build the firmware against a bench supply and have an electrician land the mains side.

What you'll need

• An ESP32 dev board (any common DevKit variant).
• A PZEM-004T v3.0 with its split-core CT coil — the version with the Modbus serial interface.
• A 5V supply for the ESP32, an enclosure, and a fused tap off the circuit you're metering.
• The Arduino IDE with the ESP32 board package, the Nodrix library, and the PZEM004Tv30 library, both from the Library Manager.
• A nodrix instance with a project and a project token.

Why use dedicated metering hardware

The classic DIY route — an SCT-013 current clamp into the ESP32's ADC with EmonLib — measures only current and assumes a mains voltage to estimate watts. Real mains sags and swells a few percent all day, the ESP32's ADC is famously nonlinear, and reactive loads (fridges, motors, anything with a power supply) make apparent and real power diverge.

The PZEM-004T v3.0 samples voltage and current together in purpose-built metering silicon and hands the ESP32 six finished numbers over serial: volts, amps, watts, kWh, hertz, and power factor. It measures 80–260V, up to 100A with the external CT, and keeps its energy count through power cuts. The ESP32's job collapses to what it's good at: asking for numbers and shipping them to the cloud.

Wiring

Two sides, kept physically apart. The mains side: line and neutral into the PZEM's voltage terminals (through the fuse), and the CT clipped around the line conductor. The low-voltage side is four wires:

Power the PZEM's interface side from the ESP32's 3.3V pin, not 5V. The comms side is optically isolated from the mains-side metering chip and runs happily at 3.3V — and it keeps the PZEM's TX at levels the ESP32's RX pin (which is not 5V-tolerant) is built for. If your particular module only talks when powered at 5V, keep the 5V supply but put a two-resistor divider on the PZEM-TX line.

The firmware

One socket carries everything: readings go up, and the connection stays open for anything you add later (a relay, a counter reset). The nodrix Arduino library owns the socket and the reconnects; the sketch is just a read-and-send loop.

#include <Nodrix.h>
#include <PZEM004Tv30.h>

const char* WIFI_SSID = "your-ssid";
const char* WIFI_PASS = "your-password";
const char* HOST      = "nodrix.you.workers.dev";
const char* TOKEN     = "tok_your_project_token";

PZEM004Tv30 pzem(Serial2, 16, 17);

void setup() {
  Nodrix.begin(WIFI_SSID, WIFI_PASS, HOST, TOKEN);
}

void loop() {
  Nodrix.run();

  static unsigned long lastReading = 0;
  if (millis() - lastReading >= 10000) {
    lastReading = millis();

    float voltage = pzem.voltage();
    float current = pzem.current();
    float power   = pzem.power();
    float energy  = pzem.energy();

    if (isnan(voltage)) return;   // meter not answering — skip, don't send garbage

    Nodrix.send("voltage", voltage);
    Nodrix.send("current", current);
    Nodrix.send("power", power);
    Nodrix.send("energy_kwh", energy);
  }
}

Worth understanding rather than copying:

• The isnan check matters. When the PZEM sees no mains voltage (breaker off, loose tap) it returns NaN for everything. Skipping the send keeps the chart honest — a gap reads as "meter offline", a stream of zeros reads as "house off", and those are different diagnoses.
• The kWh counter lives in the meter. pzem.energy() is a lifetime total that survives reboots and outages on both sides. Report it as-is; derive per-day or per-billing-period numbers in the cloud from the stored series.
• Ten seconds is a deliberate rate. Fast enough to catch a kettle, slow enough to be free on your own infrastructure — and a rate that monthly-capped hosted tiers can't sustain.
• Pin TLS before you ship. Nodrix.begin() connects encrypted but unverified on first run; add Nodrix.setCACert() for production, covered in Connect an ESP32 over HTTPS.

Build the dashboard

Four widgets, each bound to a variable the firmware is already sending:

The chart is where the project pays for itself. A day of household load has a shape — the overnight baseline, the morning spike, the compressor sawtooth of a fridge. Once you know your baseline, two
things become obvious: what's always on (that's the number that dominates the bill), and anything new that shouldn't be. A baseline that steps up 60W and stays there is how you find the amplifier
that never sleeps.

Add the alerts

Two automations cover the useful cases; both are edited in the dashboard, not the firmware.

Load spike. Trigger on a new power reading, condition above 3000 (tune to your circuit), action: Telegram — "Drawing {{value}}W right now." A washing machine tripping this at 2 p.m. is routine; a space heater tripping it at 3 a.m. is worth a message.

Meter offline. A schedule trigger every morning with an if-variable condition on voltage — if the latest reading is stale or missing, something upstream is off: breaker, fuse, or the meter itself. A monitor that fails silently is worse than none.

Swap Telegram for Slack, Discord, or SMS without touching the conditions — the alert channel is a detail of the automation, not the build.

What it costs to run

Four variables at six readings a minute is roughly a million updates a month. On hosted maker platforms that's deep into paid territory — monthly message caps are exactly what continuous monitoring burns through. Here the meter reports to your own Cloudflare account, where that volume sits comfortably inside normal Workers usage; there is no per-device fee and no message quota to manage. The economics are the point: an energy monitor only earns its keep if it runs continuously
for years.

Going further

• Switch loads from the same board. A relay on a spare GPIO plus a NODRIX_WRITE handler turns the meter into a metered smart switch — the pattern is in Receive commands on an ESP32.
• Meter more circuits. Additional PZEM units share one serial bus with distinct Modbus addresses, so one ESP32 can report power_lights, power_kitchen, power_ac — each auto-creates its own variable and chart series.
• Compute the bill. Pull the energy_kwh series from the read API and multiply by your tariff in a spreadsheet or script — the data is yours, behind one token.
• Track power factor. The PZEM also reports pf() and frequency(); two more Nodrix.send lines if you want them.

Notes

• Nothing here can be deprecated on you. The meter speaks Modbus, the board speaks HTTPS and WebSocket, and the platform is open source (MIT) on your own account.
• The data is queryable. Every reading lands in your tenancy and comes back out through the read API — no export button to hunt for.
• Configurable without reflashing. Thresholds, alert channels, and message text all live in the automation editor.

FAQ

Why a PZEM-004T instead of an SCT-013 clamp and EmonLib?

The PZEM-004T measures voltage, current, power, energy, frequency, and power factor in dedicated metering hardware and hands the ESP32 finished numbers over serial. The SCT-013 route reads a raw current waveform on the ADC and estimates power by assuming a fixed mains voltage, so it drifts with every sag and it can't see power factor. The clamp still has one advantage — it's fully non-invasive — but for numbers you'd bill against, the PZEM is the right tool.

Does this work on both 230V and 110V mains?

Yes. The PZEM-004T v3.0 measures 80–260V AC at 45–65Hz, which covers 110V/60Hz and 230V/50Hz systems alike. The firmware doesn't change — the meter reports whatever it sees.

Where is the kWh total stored, and does it reset when the board reboots?

The energy counter accumulates inside the PZEM itself and survives both ESP32 reboots and power cuts. The firmware just reports it. If you want billing-period totals, keep the lifetime counter as-is and compute deltas in the cloud — or call the library's resetEnergy() from a control write when a new period starts.

Is it safe to build this myself?

The low-voltage side — ESP32, serial wiring, USB — is as safe as any breadboard project. The mains side is not: the PZEM's screw terminals connect to live line and neutral. Work with the circuit de-energized, put everything in an enclosure so no live terminal is exposed, fuse the tap, and if you're not comfortable working inside a mains box, stop at a plug-in smart meter or ask an electrician. The CT coil itself clips around one insulated conductor and never touches copper.

Can the same build switch the load on and off?

Yes — add a relay on a spare GPIO and a NODRIX_WRITE handler for a switch variable, the same downlink pattern as the toggle in the smart-home guide. Size the relay for the load and keep it on the mains side of the enclosure. The meter and the switch stay independent: you can monitor without switching, or switch without trusting the meter.

How much data does reporting every 10 seconds generate?

Four variables every 10 seconds is about 34,000 updates a day. That's nothing for a WebSocket on your own Cloudflare account, but it's exactly the kind of rate that burns through hosted-platform free tiers with monthly message caps — one reason energy monitors are usually the first project to outgrow them.

Originally published on nodrix.live. nodrix is an open-source (MIT) IoT platform you deploy to your own Cloudflare account — source on GitHub.