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How To Read Power Usage From a Power Meter

A lot of people want to know the power usage of various electronic devices. If the device that you want to measure is permanently wired in to your home such as a hot tub, stove, well pump or water heater, then one way that you can determine how much power it uses is to read your power meter. This is a good method for determining how much power the following kinds of devices use:

  • Heat Pump
  • Electric Heat
  • Water Heater
  • Dishwasher
  • Stove or Oven
  • Hot Tub
  • Well Pump
  • Irrigation Pump
  • Welder
  • Outbuildings

Also note that this method is not accurate for low power devices such as televisions, radios and computers. It is only effective for large power devices like the ones listed above. If you want to know how much power a device that plugs into the wall uses, please see our How To Measure Watts guide..

You will need to find a time of day when you can control what devices in your home are on or off, and be aware of devices that turn themselves on and off automatically such as water heaters, refrigerators and electric heat. The most accurate method is to turn off all breakers in your house except for the ones controlling the device that you want to test. This way you ensure that only that device you want to test is drawing electricity during the test. If you want to test your hot tub, then turn off all breakers except your hot tub breaker. You might want to do this when you are the only person home.

Here is a general outline the process that you will use, regardless of what kind of meter that you have:

  • Turn off all devices that may turn on or off automatically such as heaters and refrigerators, or turn off all breakers except the one powering the device that you want to measure.
  • Turn on the device that you want to measure. Note the power usage at the meter of your entire home with the device that you want to know about turned on.
  • Turn the device off and note the power usage in your home again.
  • The difference between those two readings is the amount of power that the device uses.

There are two kinds of meters in common use today: Analog Power Meters, and Digital Power Meters. We will show you how to read each.

How To Read Power Usage From an Analog Power Meter

You will need a stop watch for this. Every analog meter has two things in common:

  1. A spinning wheel with a mark on it that you can watch.
  2. A Kh rating printed on the front

Make a note of your meters Kh number. In the meter pictured below it is circled in red and is 1.8Kh, which is a very common Kh value. In fact, on analog meters the Kh value is usually a multiple of 1.8 such as 3.6 and 7.2. This falls out of the fact that there are 3600 seconds in an hour.

To determine the power being used by your house, use your stopwatch and time exactly how long it takes for the wheel to make 1 revolution. Then use the following formula:

(3600 * Kh) / Seconds Measured = Watts used

(3.6 * Kh) / Seconds Measured = kiloWatts used

We'll do an example using the steps outlined above. Lets say you want to measure how much power your AC unit uses, and your meter has a kH of 1.8. You don't want to turn off breakers, so you unplug your refrigerator and make sure no other automatic devices are turned on. Then turn your AC unit on. You time the wheel and at takes 5 seconds to make a revolution. Therefore:

(3.6 * 1.8) / 5 = 1.3 kilowatts with the AC on

Now you turn the AC off and time the wheel again. This time it takes 20 seconds to make a revolution.

(3.6 * 1.8) / 20 = 0.32 kilowatts with the AC off

So the total power used by the AC unit is:

1.3 - 0.32 = 0.98 kilowatts, or 980 watts

You can get more accurate results by timing more than one revolution of the wheel, then divide your answer by how many revolutions you timed. Don't forget to plug your refrigerator back in.

How To Read Power Usage From a Digital Power Meter

You will need a stop watch for this. Digital power meters are relatively new, so there is not as much information about how to get instantaneous readings out of them yet.

All of the digital power meters that we have seen have a digital wheel on them. This may or may not simulate the spinning wheel that is found on analog power meters, but it will blink in some pattern that you can determine. They also have a Kh rating printed on the front. In the picture below the digital wheel is circled in blue, and the Kh rating is circled in red, and is 1.0Kh.

On Itron style power meters like the one pictured above, the digital wheel has 3 segments. Each segment counts as a complete revolution.

Do not time digital wheel revolutions, time digital segments turning on or off.

After you figure this out, the math is the exact same as for analog meters above.

(3600 * Kh) / Seconds Measured = Watts used

(3.6 * Kh) / Seconds Measured = kiloWatts used

The thing that makes digital meters really easy is that the Kh value is usually 1.0. Using the meter above with a 1.0Kh rating, if you timed each segment as turning on or off at 3 seconds, then your power usage is:

(3600 * 1.0Kh) / 3 seconds = 1200 watts, or 1.2 kilowatts

You will have to experiment a little to make sure that your readings make sense and that you are reading the correct blinking dot. Try measuring the power usage of a device who's usage you already know and see if your answer is in the correct ball park. A good test might be your microwave, because they frequently draw about 1000 watts. If you do the math and come up with your microwave drawing 15,000 watts then you know you are doing it wrong. Likewise if you do the math and come up with your microwave drawing 100 watts, then you are still probably doing it wrong. If you do the math and determine that your microwave is using 1150 watts then you are probably doing it right.

You may want to read our guide on How To Determine What you Pay for Electricity.

Need help? Ask in the forum.

 

How To Read Power Meters
Jason Bauer

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Jason Bauer is an owner and programmer for Portforward.com. He's allergic to twitter and facebook, but you can find more of his articles in the Guides section.
 
Sunday, 20-Apr-2014 16:45:21 PDT