Kill A Watt

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:

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

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:

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 $30 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 2023. These are ordered based on current price, lowest to highest. All are rated 4+ stars on Amazon.

TechBee Electricity Usage Monitor ($11): Provides detailed information on power (wattage), amperage, voltage, frequency, power factor and CO2 emissions. It will provide usage details over a time period for kWh used, carbon emissions, and total cost.  Comes with a disposable coin battery (to preserve its memory of consumption in case of a power outage).

One drawback of this meter is that it has 11 display modes in total, and to get between modes you have one button that cycles through all 11 of them, so if you get to the mode you want and accidentally press the button one time too many, you have to press it another 10 times to get back to what you wanted. Also, the screen is not backlit and is somewhat dim so you may need to shine a light on it in dim light to get a reading.

Allows you to set the unit price for electricity, but does not allow you to set the carbon emissions per kilowatt, so if you want to know your actual carbon emissions and you live in a grid where much of the eletricity is non-carbon-based, you’ll need to adjust accordingly. All in all not a bad unit considering the price.

Mecheer Upgraded Watt Power Meter ($12): This device can measure power (Watts), energy (kWh), volts, amps, hertz, power factor, cost, minimum and maximum power (the minimum wattage and maximum wattage used during the period a device was plugged in – the maximum is useful to determine the peak usage of an appliance.

Unlike the TechBee, the Mecheer has a backlit display so you won’t struggle to read it in dim light. While it doesn’t have a battery, it does have a memory so that even during a power outage it maintains information on electricity consumption.

Allows you to set the cost of electricity. Does not provide any information on carbon emissions. Also, you can’t easily determine watt-hours used per day accurately, because when it gives a watt-hours used count and a time period, it only shows number of days, not fractional or hours within days.

Towalmark Power Consumption Monitor ($13): This monitor can measure voltage, amps, watts, kilowatts consumed over time, and cost over time. It does not provide any information on carbon emissions. The display is backlit, although it is hard to see from above, and given that many power outlets are close to the floor this creates challenges in getting a reading.

The unit has an internal battery so can store consumption over time, as well as minimum/maximum watts over the time period, Allows you to set a cost per kWh, but the process is convoluted. Another drawback is that if you want to reset it to restart the time count or the minimum/maximum watts, this also resets the cost per kilowatt.

The documentation for this electricity monitor is very basic, so you may find it challenging to figure out how to use some of the more complex functions such as setting the electricity price.

BN-LINK LCD Plug-In Power Meter ($14): Provides 9 display modes: Watts, VA (Volt/Amperes – which is nominally but not exactly equivallent to Watts), kilowatt hours consumed, elapsed time, frequency (Hz), power factor, Volts, Amps and cost. The display is very basic showing only one value; there are buttons for each of the modes so you can quickly view the mode you’re interested in. One problem is you can’t usually tell which mode you are viewing so you may need to press the appropriate mode button again to figure that out. Does not include carbon emissions.

The unit includes a battery so that you can unplug it and move it to a new outlet and it will continue to track time and consumption. Resetting it is awkward, requiring you to reset each digit individually to zero.

My view on this unit is that its very simple display may quickly make you wish you had gone for one of the more versatile (and typically somewhat cheaper) models above. Its ratings are similar but you have to bear in mind that most people only buy one unit, and people seem happy with this one.

Poniie PN1500 Portable Micro Electricity Usage Monitor ($15): This unit has similar functions to the units above – kilowatt hours consumed, volts, amps, watts or VA, Hz, Power Factor and cost. It has five buttons; each button has two functions (for example, the Volt button is also the UP button, and the Amp button is also the Down button).  It has a large LCD display that is easy to read from different angles. The display is continuously backlit, which is convenient in that it will remind you to glance at it as you pass by, but you wouldn’t want to use that to measure the consumption of a device in your bedroom if sources of lighting keep you awake. The unit is smaller than the Kill-a-watt meter and the lower buttons are close enough to the plug outlet that a large plug may interfere with them.

The directions are not great – they are very technical, and yet even to the electrically minded not helpful. Definitions of the different measurements are circular – each is defined in terms of one or more of the others. On the other hand, people do not seem to have any trouble figuring out how to use the device and make use of the different measurements, even if the manual doesn’t explain them well.

KETOTEK  Electricity Usage Monitor ($17): Similar to the Poniie 1500, this unit has multiple functions for each of its five buttons, although it looks completely different (I’d give the Ketotek the win on style). The display, on the other hand, shows more details – you can for example see duration, kWh used, and cost, all on one screen. It can measure Watts, kWh, voltage, frequency (Hz), minimum and maximum power draw, electricity cost, VA (apparent power), Amps (true power), power factor and cumulative time.  The display is also easily viewed from different angles

This unit like many others can store your data through a power failure, although it does not use a battery. It provides visual warnings for wattage overload (where you are drawing more than the default 3680W; this requires some setup beforehand); kWh used and cost used (in other words, you can set a number of kWh or a dollar cost as a target, and the screen will flash and display the particular alarm when you reach that threshold). Setting the electricity cost is relatively easy – just hold down one of the buttons for three seconds to enter the cost setting mode, and move the cost up or down with the up / down buttons.

Poniie PN2000 Plug-in Kilowatt Electricity Usage Monitor ($26): One of the annoyances of most electricity usage monitors is that if you want to measure consumption of a device that is plugged into a power bar or outlet where other devices are also in use, the monitor tends to cover up several other outlets. One nice thing about the Poniie PN2000 is that it comes with its own short power cord so that you can plug the monitor into the power cord, and plug the cord into the outlet, without blocking any other outlets. (Of course, you could just use an extension cord for this!). Other than that, features it stands out for are a high power tolerance (some of the cheaper models cannot handle heavier loads – this one can go up to 16 Amps, and can be used on most Level 1 car chargers to measure consumption), and sensitivity to low power modes (detects power draw as low as 0.2 watts, meaning you can use it to measure consumption of a small LED bulb or a phantom load on a charger). It does not provide either cost measurement or carbon emissions measurement.

Unlike many of the other electricity usage monitors mentioned above, the Poniie PN2000 comes with high quality comprehensive documentation, but even if it didn’t, it is simple enough to figure out how to use it. You’re definitely paying quite a bit more for this unit than some of the items further up this list – the main reason to do so would be quality and accuracy of the readings, as the unit is very well built and some of the other units may not provide fully accurate readings. (But any of them will still provide useful information about where your electricity dollars are going, even if they’re off by 1-2% and therefore not giving you a fully accurate wattage or voltage.)

Poniie PN2500 Professional Wireless Level 1 EV Charger Pass-Through Power Usage monitor ($60): This power monitor is great for you if you want to measure power consumption on your Level 1 EV charger while charging your car on a standard Nema 5-15 electrical circuit. It’s safe to use at up to 25 amps or 3000 watts, which is more than a typical 110V circuit is wired for. It provides the same metrics as other electricity usage monitors including Watts/VA, kWh, Time, Amps, Start-up Amps, Voltage, Power Factor, Hz and Cost (note that the cost setting is configurable but for some reason Poniie chose to limit the cost to $0.50 per kWh, which is below the full rate in some jurisdictions). And unlike all of the cheaper monitors listed before, it includes WiFi support (2.4 GHz only – meaning it does not support the 5GHz frequency of most routers) and a smartphone app so you can connect it to your iOS or Android phone or other device. This app is useful if you want to periodically check usage (or consumption as you charge your car) or if the device is in a hard-to-reach space like the wall behind a refrigerator. The large LCD screen is backlit so it is easy to see in any lighting condition or from any angle, although the smartphone app makes that feature less important than for the other monitors above.

The main reasons to buy this considerably more expensive monitor over something basic like the Kill-A-Watt meter or the Poniie PN1500 are the WiFi connection and smartphone app, the higher power rating that makes it usable for heavy loads and Level 1 car chargers, and the fact that it can measure peak load on startup, which is important for some applications such as system sizing where multiple devices might need to start up all at once (for example after a power outage).

Any of the above electrical usage monitors should serve you well. All get high reviews. Even the cheapest ones do what they’re designed to do; the main drawbacks on some of those units are the difficulty of reading the display, and a challenging user interface. For the display it’s relatively easy to solve this in low-amperage measurements by using an extension cord between your outlet and the monitor, and then plugging whatever device you’re measuring into the monitor; that way the monitor can be placed in a place that makes it easy to read. If you are curious about your car charging usage, or like the luxury of a phone app to keep checking on your usage, go with the Poniie PN2500, but otherwise you can try any of the devices on this page to get a good sense of where your power is going. Once you have one and are looking for more ideas on finding cost savings, check out my page How to Save Electricity for lots of helpful tips.