How to measure power in PID controlled circuit

Projects General Guidance
7 
#include <PZEM004Tv30.h>
#include <PID_v1.h>
#include <Wire.h>
#include <Adafruit_MLX90614.h>

//Solid state relay on PIN 5 on the Arduino
#define RELAY_PIN 5

//Define Variables we'll be connecting to
double Setpoint, Input, Output;

//Specify the links and initial tuning parameters
double Kp=1500, Ki=.5, Kd=20;
PID myPID(&Input, &Output, &Setpoint, Kp, Ki, Kd, DIRECT);

int WindowSize = 10000;
unsigned long windowStartTime;

Adafruit_MLX90614 mlx = Adafruit_MLX90614();


PZEM004Tv30 pzem(11, 12);// connection to Power sensor Tx-11 and Rx-12

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, LOW);
  Serial.begin(115200);
  Serial.println("Starting");
  windowStartTime = millis();

  //initialize the variables we're linked to
  Setpoint = 100;

  //tell the PID to range between 0 and the full window size
  myPID.SetOutputLimits(0, WindowSize);

  //turn the PID on
  myPID.SetMode(AUTOMATIC);
  mlx.begin();
}

void loop() {
  Input = (mlx.readObjectTempC());
  Serial.println(Input);
  myPID.Compute();

  /************************************************
   * turn the output pin on/off based on pid output
   ************************************************/
  if (millis() - windowStartTime > WindowSize)
  {
    //time to shift the Relay Window
    windowStartTime += WindowSize;
  }
  if (Output < millis() - windowStartTime) 
    digitalWrite(RELAY_PIN, LOW);
  else 
    digitalWrite(RELAY_PIN, HIGH);
  

  Serial.println();
  delay(0);
    float voltage = pzem.voltage();
    if(voltage != NAN){
        Serial.print("Voltage: "); Serial.print(voltage); Serial.println("V");
    } else {
        Serial.println("Error reading voltage");
    }

    float current = pzem.current();
    if(current != NAN){
        Serial.print("Current: "); Serial.print(current); Serial.println("A");
    } else {
        Serial.println("Error reading current");
    }

    float power = pzem.power();
    if(current != NAN){
        Serial.print("Power: "); Serial.print(power); Serial.println("W");
    } else {
        Serial.println("Error reading power");
    }

    float energy = pzem.energy();
    if(current != NAN){
        Serial.print("Energy: "); Serial.print(energy,3); Serial.println("kWh");
    } else {
        Serial.println("Error reading energy");
    }

    Serial.println();
    delay(500);
}