This page is dedicated to showing you how to perform what is know as a watt-load check. This is also referred to as the time-watt method. What we will be doing, in essence, is learning how to estimate the load on our service. This method is used by meter techs to verify that the meter is running properly. If you learn how to do this simple calculation you could be well on your way to saving money on your power bill.
The things that you will need to know to perform the watt-load check and estimate the load on your service are the formula below, a stop-watch, and you will need the Kh of the meter that you will be performing the watt-load check on. If you will remember from the watt-hour meter basics page, the Kh is found on the meter nameplate. It is normally a number such as, 7.2, 3.6, 1.0, 10.0, 14.4 and 1.8. There are more, but this should point you in the right direction.
Here is the formula:
Kh x 3600 x number of revolutions
time in seconds
So, if we take the meter below and find the Kh, we can start the process.
.If you will notice, the Kh is 1.8. If you will also notice, in the middle of the meter where the round disk is coming through the register, there are two vertical black lines. You cannot see it in this picture but there is a black mark on the disk as well.
Now that you have identified the Kh, the next step to estimate the load on the service is to determine how many revolutions that you would like to let the disc rotate. Ten revolutions is a good easy number to use in calculations so we will pick ten. The next step is to wait until the black mark on the disk passes by the vertical black lines on the meter. At this point, you want to hit start on your stop-watch.
Then you just wait until the tenth time that the black mark passes the vertical lines and hit stop on your stop-watch. Let’s just assume that it took 25 seconds to make ten revolutions. Now you have everything that you need to make the calculation to estimate the load on your service.
So, I know that sometimes we forget our grade school math so I am going to multiply the top of the fraction first, get the answer to that problem and then divide it by the time in seconds.
1.8 x 3600 x 10 revolutions = 64,800
64,800 / 25 seconds = 2,592
The next question is, what is this telling us? 2,592 is the number of watts that were consumed during the 25 seconds that we did the test. Remember however that we are billed on kilowatt hours. To get kilowatt hours we need to divide 2,592 by 1,000. That equals 2.592 KW. So, if you were to continue this for one hour, you will have consumed 2.592 KWH.
How do you know that what you just did was accurate? You can convert the 2,592 watts to amps and check it. The only caveat being that you will have need to have an ammeter either in the meter base checking the amps while you are doing the test, or in your panel on the main feeder. Only attempt this if you are qualified to do so.
To convert watts to amps we use part of Ohm’s Law. Ohm’s Law says that Watts = Volts x Amps. Since we know the watts, we can plug that in. The voltage will be determined by whatever the line to line service voltage is. In most residential cases, the voltage will be 240v. However, in the case of the meter that I am using for an example, the voltage is 120v.
2,592 watts = 120v x amps
To get amps, we divide both sides by 120v.
2,592 watts / 120v = 120v / 120v x amps
21.6 = amps
So, in this case the service had 21.6 amps on it when we checked it. Knowing this can help you track down what is using all of your power. In this case, it could be a water heater or a window unit. If you can go through this calculation and look at your meter you will easily be able to estimate the load on your service.
I hope this has helped someone. By utilizing this simple calculation, you will be well on your way to saving energy and saving money on your power bills.