I paid $60 for my Kill-A-Watt meter about two years ago now, and even though it cost me about three times what they sell for now, it has paid for itself several times over in electricity savings.

The Kill-A-Watt meter measures your electricity usage at a power outlet. Here’s how this amazing electricity monitor works:

  • You plug the meter into an outlet. The key is that the meter needs to be visible when you’re doing a measurement, so for instance if you are measuring refrigerator consumption, don’t hide the meter behind the fridge. In this case, use an extension cord.
  • The electricity monitor has a plug on its front, that you can use to plug in either a single device, or an extension cord or power bar with several devices attached. Just make sure you don’t exceed the 15 amps maximum for the meter, which means you shouldn’t plug in a load of more than 1,800 watts.
  • As soon as the meter is plugged into a power outlet, it starts measuring both time and kilowatt hours of electricity consumption. Whenever a device is plugged into it, it can display the watts consumed by the device at that time.

In terms of measuring consumption, I like to think of four types of devices, based on their pattern of electricity use: continuous load devices (such as a lightbulb, which always burns the same amount); phantom or vampire loads (which use small amounts continuously, often without doing you any good; cycling loads such as a refrigerator or freezer, where you want to know how much is used over a 24-hour period; and very low loads like LED Christmas lights, which use so little electricity it’s hard to measure!

Continuous load devices

If you are interested in the constant load of a device (how much it draws whenever it’s in use), you can use the watt reading for a few seconds to determine how much current the device draws. You can then multiply that wattage by the number of hours the device is typically in use in a day or a month, to estimate the watt hours the device consumes. Divide by 1000 to get kilowatt hours.

For example, if you wanted to measure the electricity use of a toaster, plug the toaster into the killawatt meter, which is still not plugged into the wall socket. Put the toast in the toaster. Then quickly plug in the electricity energy monitor and push down the toast slider at the same time. As the toast is cooking, press the red button on the meter until you get the Watts reading. The reading may fluctuate but not by very much. Then press the red button again to switch to the time view. When the toast pops up, you have a time measurement in minutes and seconds, and a watt measurement. Use this formula to calculate kilowatt hours used by your toaster:

Minutes used × watts
60 × 1000
= kWh

So, if it takes you 3m 12s to toast two slices of bread, and the toaster used around 750 watts, you would calculate:

3.1 minutes × 750 watts
= 0.03875 kWh

Then multiply by the number of times a year you make toast, to get kilowatt hours per year consumed by your toaster. In my household, I would guess that we use the toaster about ten times a week, or 523 times a year. So for the above example, I would guess our toaster uses 20 kilowatt hours per year.

Phantom load devices

For some phantom loads (also called vampire loads or electric vampires), the Kill-A-Watt meter can quickly tell you how much power you are wasting on the load. For example, plugging in a cable modem power converter will quickly tell you how many watts the converter uses in its steady state. In my case, the reading was 9 watts.

For very low wattages (1-4 watts) you will get an imprecise reading, since the meter does not measure anything smaller than a watt, which means your reading of 3 watts could mean anything from 3.00 watts to 3.99 watts. But that 0.99 watts is only 8.75 kWh per year, which is probably not enough to change how you use the information.

Don’t forget to measure phantom loads such as devices with digital clocks that are otherwise not doing anything useful, such as coffee makers, video players, and bread makers. Also, don’t forget to measure how much the coffee maker uses when it’s just keeping coffee warm.

Cycling loads

For cycling devices such as refrigerators and freezers, you need to measure both time and kilowatt hours over a number of days in order to get a balanced reading. A short measurement may be skewed by the cyclical nature of such loads. A three day or longer reading is good. Be aware that the hour reading wraps at 100 hours, so make note of when the measurement started. Periodically read both the kilowatt hours consumption and the elapsed hours. Divide the kWh by the hours and multiply by 24 to get kWh consumption per day. Initial kWh readings may vary widely. The longer you keep measuring, the more the kWh per day calculations should converge.

Also be careful not to unplug the Kill-A-Watt meter while measuring cycling use. The meter has no battery, so unplugging it effectively erases any prior measurement and restarts the count of kilowatt hours and elapsed time to zero the next time the meter is plugged in.

Here is the formula to calculate kilowatt hours per day once you’ve measured elapsed time and kilowatt hours through that time:

kWh reading × 24
elapsed hours
= kWh per day

Very low loads

If you’re interested in measuring the load of a device that consumes almost no power, such as a string of LED outdoor lights, the wattage reading is not precise enough. I tried measuring one short string of LED lights and the load I got was 0 watts. For such situations, leave the load plugged into the Kill-A-Watt meter for long enough that at least two non-zero rightmost digits appear in the kilowatt hours reading. Then divide by the time in hours. Doing this kind of measurement won’t make much of a dent in your electricity bill, but you may find it thrilling, and motivating to your friends, to tell them that the LED lights you use to light your front yard at night use only 1.7 watts of electricity!

Spread the wealth

Once you’ve gotten your mileage out of your Kill-A-Watt meter, lend it to friends, family, neighbors. But don’t just give it to them; offer to measure their usage for them. Here’s why I recommend this:

  • It’s easier to show someone how to use the meter than to explain the process.
  • It’s more likely they will start serious measurements and will follow up with energy saving choices, if you help them get started
  • I’ve had too many people borrow my meter week after week, only to find out after repeated nagging that they just never got around to doing any measurements. In fact, my Kill-A-Watt meter has been at a friends’ house for ten months now – time to e-mail them and ask them to bring it over! Hopefully he has been lending it out to others.

If you decide you don’t need your meter any more – if you’ve already measured everything and made your energy saving choices based on what you found – donate your meter to your local library, or any other group that lends such meters out. If you are into saving energy partly because you care about the environment, donating your meter will mean many more households will achieve the same savings and reduce their negative impacts on the environment.

Other electricity usage meters

I’ve written extensively about the Kill-A-Watt meter because (A) I’ve used it a lot myself and had great results, (B) it’s one of the most inexpensive on the market, selling online for around $25 US, and (C) it seems to be the most popular, based on how often it pops up on web searches.

But there are a number of other good electricity usage meters on the market, in different price ranges. I have not used any of these but based on user reviews and descriptions of features the following sound promising. I’ve included online pricing as of October 2008:

The Energy Detective (TED): Full-house electricity energy monitor including cost of current energy use. Requires installation of one component on your circuit breaker panel, by an electrician or experienced homeowner. The second component is plugged into any wall socket and provides a read-out of whole house consumption, so you can instantly see the changes that occur when a particular appliance or light is switched on or off. Costs around $185 US.

Watts Up AC Power Meter: Similar to the Kill-A-Watt meter but includes the ability to enter a price per kilowatt hour in tenths of a cent so that you can see the cost of using each device, and automatically calculates kilowatt hours per month for a device that has been plugged into it. Also has an attached power cord so you can plug the meter directly into a socket behind an appliance, and still see it (for the Kill-A-Watt meter you need to use an extension cord). Has some extra features like the ability to capture the maximum power use of a device. Costs around $110 US.

The Watts Up Pro pictured at right has even more bells and whistles and goes for a little bit more.

PowerCost Monitor: Another full-house electricity energy monitor. Does not measure individual device consumption separately. Exterior unit attaches to your power meter and optically reads power consumption from the spinning power dial (analog meters) or optical port (digital meters). Inside unit can be placed anywhere within 30 meters of the electricity meter. Includes several pricing structures including single-rate (same $ per kWh all the time), tiered (different rates based on how much you consume per month, as in Ontario Canada), and time of use (different rates based on the time of day used). Costs around $135 US.

Kill-A-Watt EZ: This version of the Killawatt meter adds some extra features such as: you enter your electricity rate; it estimates costs by week, month and year; and it has a battery backup, so that if the power cuts when you’re trying to measure kilowatt hour consumption on a cycling or intermittent appliance (for instance a refrigerator), the Kill-A-Watt EZ can remember how much energy was consumed and how much time has elapsed. Costs around $35 US.

Plug-In Mains Power and Energy Monitor is a 240 volt electricity energy monitor similar to the Kill-A-Watt meter, intended for use in countries with 240 volt current such as the UK. Costs around $17 US (£10).

Black and Decker EM100B: The EM100B is another whole-house electricity monitor that tracks electricity use minute by minute. You can display your energy consumption in kilowatt hours and in dollars based on the electricity rate you enter. The monitor can also project your monthly bill so if you’re on a tight budget, you’ll know when to cut back. It also includes an Appliance Mode that can isolate the consumption of individual electrical devices or appliances, so you can find where all that electricity is gong.

The wireless system includes a weatherproof sensor that you can attach to your outdoor meter. It should work with most types. The sensor transmits information to the wireless display unit, which you can read from wherever it’s convenient. The system requires four AA batteries, and it’s covered by a 2-year warranty.

1 reply
  1. John Ertiat
    John Ertiat says:

    When measuring very low wattage devices with a kill A Watt that often show zero watts, you can often get a better estimate of the watts if you look at how many volt amperes (VA) it is using and multiply it by the power factor (PF) because PF x VA = Watts.


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