ESP32 PZEM-004T energy monitor

// PZEM-004T energy monitor on an ESP32.
//
// The PZEM-004T v3.0 talks Modbus-RTU at 9600 baud. Here it is on Serial2,
// the ESP32's UART2: the meter's TX goes into GPIO 16 (RX2) and its RX comes
// from GPIO 17 (TX2), and the PZEM004Tv30 library does the Modbus. The meter's
// TTL side runs from 3.3 V, so its TX swings to the ESP32's own logic level.
//
// The meter's mains side is set on the part's sliders here. On the bench it
// is wired to live mains, which can kill: only a qualified person should
// connect one.

#include <PZEM004Tv30.h>

PZEM004Tv30 pzem(Serial2, 16, 17);  // RX2 (the meter's TX), TX2 (the meter's RX)

void setup() {
  Serial.begin(115200);
  Serial.println("PZEM-004T energy monitor");
  // The library skips any read in the first 200 ms after reset (it returns
  // 0), so give it that long before the first one.
  delay(250);
  Serial.print("Meter address: 0x");
  Serial.println(pzem.readAddress(), HEX);
}

void loop() {
  float voltage = pzem.voltage();
  float current = pzem.current();
  float power = pzem.power();
  float energy = pzem.energy();
  float frequency = pzem.frequency();
  float pf = pzem.pf();

  if (isnan(voltage) || isnan(current) || isnan(power) || isnan(energy) || isnan(frequency) || isnan(pf)) {
    Serial.println("Error reading the meter: is it on the mains?");
  } else {
    Serial.print("Voltage: ");   Serial.print(voltage);      Serial.println(" V");
    Serial.print("Current: ");   Serial.print(current);      Serial.println(" A");
    Serial.print("Power: ");     Serial.print(power);        Serial.println(" W");
    Serial.print("Energy: ");    Serial.print(energy, 3);    Serial.println(" kWh");
    Serial.print("Frequency: "); Serial.print(frequency, 1); Serial.println(" Hz");
    Serial.print("PF: ");        Serial.println(pf);
  }
  Serial.println();
  delay(2000);
}