import time
import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)

GPIO.setup(26, GPIO.IN, pull_up_down = GPIO.PUD_DOWN)

def get_watts(sample_time):
    """Count impulses for sample_time seconds and then calculate average wattage."""
    impulses = 0
    start = time.time()
    while time.time() - start < sample_time:
        GPIO.wait_for_edge(26, GPIO.FALLING)
        impulses += 1

    # 10000 impulses per hour @ 1kW
    # =  10 impulses per hour @ 1W
    # impulses = watts * (10 * hours)
    # watts = impulses / (10 * hours)
    # 1 hour = 60 * 60 seconds = 3600 seconds
    # hours = sample_time / 3600
    # watts = impulses / (10 * sample_time / 3600)
    # watts = impulses / (sample_time / 360)
    # watts = impulses * (360 / sample_time)

    return impulses * 360.0 / sample_time

while 1:
    print (get_watts(60), 'watts')

GPIO.cleanup()