Home energy monitor with EmonLib on an Arduino Uno

// Home energy monitor: OpenEnergyMonitor's EmonLib reads the mains voltage
// from a ZMPT101B and the current from an ACS712, and an I2C LCD shows the
// volts, amps, real power, power factor and the energy used so far.
//
//   ZMPT101B   OUT on A0, VCC 5V, GND        (its screw terminals across the mains)
//   ACS712-05B OUT on A1, VCC 5V, GND        (IP+ and IP- in series with the appliance)
//   LCD        16x2 with an I2C backpack at 0x27: SDA A4, SCL A5, VCC 5V, GND
//
// HOW IT WORKS
//   Both sensors give a wave that swings about half the supply. calcVI()
//   samples the two in turn over 20 zero crossings (10 mains cycles), takes
//   each one's middle out, and works out:
//     Vrms and Irms   the root mean square of each wave, times its calibration
//     realPower       the mean of volts times amps, sample by sample: the
//                     watts the meter on the wall charges for
//     powerFactor     real power over Vrms x Irms: 1 for a heater or a kettle,
//                     less for a motor or a phone charger
//   Watt hours add up as real power times the time between readings.
//
// CALIBRATION
//   VOLT_CAL is the ZMPT101B's ratio: mains volts per volt at OUT. Read the
//   mains with a meter you trust, then VOLT_CAL = VOLT_CAL x meter / sketch.
//   CURRENT_CAL is amps per volt at OUT: 1000 / 185 mV per amp = 5.405 for
//   the ACS712-05B (1000 / 100 = 10 for the 20 A board, 1000 / 66 = 15.15
//   for the 30 A). PHASE_CAL moves the voltage samples to line up with the
//   current ones: with a kettle or a heater plugged in, change it until the
//   power factor reads 1.00. EmonLib's own examples use 1.7 for their AC
//   adapter; 1.0 is no correction.
//
// SAFETY
//   The ZMPT101B and the ACS712's IP pins sit on line voltage. Only a
//   qualified person should wire them to the mains, in a closed box. A
//   clip-on current transformer (SCT-013) with EmonLib's calcIrms() keeps
//   the current side off the wires altogether.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include "EmonLib.h"

LiquidCrystal_I2C lcd(0x27, 16, 2);
EnergyMonitor emon;

const float VOLT_CAL = 600.0;    // the ZMPT101B's trimmer ratio
const float PHASE_CAL = 1.0;    // no correction: see CALIBRATION
const float CURRENT_CAL = 5.405; // ACS712-05B: 1000 / 185
const float NO_LOAD_A = 0.05;    // below this the ACS712's noise, not a load

double wattHours = 0;
unsigned long lastAt = 0;

void setup() {
  Serial.begin(9600);
  lcd.init();
  lcd.backlight();
  lcd.print("Energy monitor");

  emon.voltage(A0, VOLT_CAL, PHASE_CAL);
  emon.current(A1, CURRENT_CAL);
  // The first window starts with the offsets at mid-scale and no supply
  // reading yet: measure once and throw it away.
  emon.calcVI(20, 2000);
  lastAt = millis();
  Serial.println("energy monitor ready");
}

void loop() {
  emon.calcVI(20, 2000);

  float volts = emon.Vrms;
  float amps = emon.Irms;
  float watts = emon.realPower;
  float pf = emon.powerFactor;
  if (amps < NO_LOAD_A) {   // nothing plugged in
    amps = 0;
    watts = 0;
    pf = 0;
  }
  if (watts < 0) watts = 0; // a sensor turned round reads negative

  unsigned long now = millis();
  wattHours += watts * (now - lastAt) / 3600000.0;
  lastAt = now;

  // 230V 1.41A PF1.0
  //  325W 0.9Wh
  int centiAmps = (int)(amps * 100 + 0.5);
  int tenthsPf = (int)(pf * 10 + 0.5);
  char line[17];
  char wh[10];
  dtostrf(wattHours, 1, 1, wh);
  lcd.setCursor(0, 0);
  snprintf(line, sizeof line, "%3dV %d.%02dA PF%d.%d", (int)(volts + 0.5),
           centiAmps / 100, centiAmps % 100, tenthsPf / 10, tenthsPf % 10);
  lcd.print(line);
  lcd.setCursor(0, 1);
  snprintf(line, sizeof line, "%5dW %sWh      ", (int)(watts + 0.5), wh);
  lcd.print(line);

  Serial.print("V ");
  Serial.print(volts, 1);
  Serial.print("  A ");
  Serial.print(amps, 2);
  Serial.print("  W ");
  Serial.print(watts, 0);
  Serial.print("  PF ");
  Serial.print(pf, 2);
  Serial.print("  Wh ");
  Serial.println(wattHours, 2);

  delay(800);
}