Simple

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class LampLife : MonoBehaviour {

    public float minimumLife = 1;
    public float powerLife = 20;
    public float counterLife;
    public float speedOfTheGame = 1.5f;
    public RawImage image;
    private Light lamp;
    public Text powerLifeText;

    private void Start(){
        lamp = GetComponentInChildren<Light>();
        lamp.enabled = false;
    }

    private void Update(){
        counterLife = speedOfTheGame * Time.deltaTime;
        if((int)powerLife <= 20 && (int)powerLife >= 10){
            image.color = Color.green;
        }
        else if((int)powerLife >= 5 && (int)powerLife <= 10){
            image.color = Color.yellow;
        }
        else if((int)powerLife > 0 && (int)powerLife <= 5){
            image.color = Color.red;
        }
        else if(powerLife >= 0 && powerLife < 1){
            lamp.enabled = false;
        }

        if(powerLife >= minimumLife && lamp.enabled != false){
            powerLife -= counterLife;
        }
        powerLifeText.text = (int)powerLife + " % ";
    }

    public void GetPowerLife(int power){
        powerLife += power;
    }
}