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A wattson energy monitor

Electrical energy monitors

by Chris Woodford. Last updated: March 18, 2013.

Is the air conditioner costing you a fortune? What about that electric fire... or the washing machine... or the dishwasher? And how can you find out? Step forward the energy monitor! Simply place it near a power cable coming from an appliance and you get an instant measurement of how much you're paying for electricity per hour. What a brilliant idea for saving your pocket and the planet! If you've seen one of these things in action (popular brands include the wattson and the Owl), you might have wondered just how they work their magic. Let's take a closer look and see!

Photo: A wattson home energy monitor. You can see from the LED display that it's currently reading 328 W (watts)—in other words, 328 joules (J) of energy is being consumed per second. Photo by courtesy of Paul Pod, published on Flickr under a Creative Commons License.

Why you need to save electricity

home electricity meter

Electricity is having a difficult time—or, rather, making it has never been more tricky. Oil and gas are rapidly running out, coal's dirty, wind power and solar panels still aren't completely competitive, nuclear worries people. Add to that the difficult problem of global warming (the way Earth is slowly heating because of carbon dioxide produced when we consume energy) and you can see we're in a bit of bother. If you're troubled by the high cost of energy, or by the effect people are having on the planet, why not start using energy more wisely? You can drive more efficiently, for example, switch off your air-con, or turn down your room thermostat a degree or two. Another good thing you can do is try to use energy-efficient lamps. But if you want to make a really big difference to your home energy consumption, you need to tackle your power-hungry appliances: your cooker, refrigerator, freezer, dishwasher, washing machine, and electric kettle. If you've no idea how much electricity you're using, that's where electrical energy monitors can help!

Photo: An ordinary electricity meter tells you the total amount of electricity you've used for all time. It's not very helpful for cutting your consumption.

How can you measure electricity consumption?

If you use electricity, you have a meter somewhere in your building that keeps a record of how much you're consuming so your utility company can bill you for it. The only trouble is, the meter measures your total electricity consumption for every appliance you're using for all time. That makes it hard to know how much you're paying to run any one of the dozens of appliances you may be using and to discover which ones are wasting energy.

Pie charts showing growth in use of home appliances and electronic gadgets between 1978 and 2005.

Now, in theory, it should be easy enough to unplug any electrical appliance, plug it into a meter of some kind, and plug the meter into your electricity outlet (that's "mains socket" to you Brits)—and there are quite a few energy monitors that work in exactly this way (including one amusingly called the Kill-a-Watt). But some appliances, especially the really power-hungry ones (refrigerators, deep freezers, and electric cookers among them) are difficult to unplug. Isn't there another way? The new electricity consumption meters that have started appearing over the last few years work totally differently—using the electricity you consume to make magnetism, turning the magnetism into electricity, and then measuring that electricity. Let me explain...

Charts: More and more of the energy we use at home is going to power gadgets and appliances. Look at these two charts, which compare US home energy use from 1978 and 2005. Blue = home heating, Orange = appliances and electronics, Yellow = water heating, Green = air conditioning. You can see that while we're using less energy to heat our homes, the share of energy use for appliances (the orange segments) has grown enormously. Data from Share of energy used by appliances and consumer electronics increases in U.S. homes, US Energy Information Administration, March 28, 2011.

The magic of electromagnetism

As you might know already, electricity and magnetism are like an old married couple: you never get one without the other. That's how all kinds of electric appliances work, from motors and generators to transformers and headphones. If you send a fluctuating electric current down a cable, it creates an invisible magnetic field all around the cable at the same time. This surprising effect was first discovered by a Danish physicist named Hans-Christian Ørsted (1777–1851) when he placed an electricity cable over a compass and switched on the power. French physicist André-Marie Ampère (1775–1836) took Ørsted's finding a step further by showing that the strength of the magnetic field is directly related to the size of the electric current: put a bigger current through the wire and you get a stronger magnetic field around it.

Let's say we have an electric toaster plugged in and we're cooking some bread. How can Ørsted and Ampère help us figure out how much our toast is costing? Consider the cable that's connecting the toaster to the power outlet. As electricity's charging down it, a magnetic field is being created all around it. So, all we have to do is measure the strength of the magnetic field: the bigger the field, the more electricity we're using.

James Clerk Maxwell

Now this is the clever part. Just as an electric current can create a magnetic field, so a magnetic field can create an electric current. It's called electromagnetic induction. Suppose you have a magnet and you move it around near a length of electric cable. If you hook up the cable to a voltmeter, you'll find that electricity flows through the cable every time you move the magnet. A changing magnetic field makes electricity flow through a conductor that's inside the field. English physicist Michael Faraday (1791–1867) found this out about 10 years after Ørsted's original discovery and that led him to invent the generator—the device that makes virtually all the electricity we use in our homes.

Photo: Scottish physicist James Clerk Maxwell (1831–1879) wrapped up the work of Ampère, Ørsted, Faraday, and others to make a comprehensive theory of electricity and magnetism. Maxwell's theory of electromagnetism is summed up in four amazingly elegant mathematical equations. Public domain photo by courtesy of Wikimedia Commons.

Now we can cut to the chase: to measure how much electricity an appliance is using, you simply place a coil of wire around (or very near to) the main cable through which the power is flowing. Let's call this coil the probe. As the electricity flows, it'll generate a magnetic field around the main cable. The magnetic field constantly fluctuates because the electricity flows rapidly back and forth in what's known as an alternating current (AC). The fluctuating magnetic field generates an electric field in the coil of wire that makes up our probe. All we need to do is wire the probe up to a meter that measures electric current. The more electricity our appliance uses, the bigger the current that will flow in our probe.

How energy monitors work

Simple line artwork illustrating how an energy monitor works

  1. You switch on your appliance.
  2. Alternating current (AC) carries electrical energy into it.
  3. The AC current flows back and forth along the power cable between the power outlet and your appliance.
  4. As the current changes, it creates a magnetic field all around the cable.
  5. Put your energy monitor's probe near the cable and the magnetic field causes a secondary alternating current to flow inside it. (The magnetic field from an ordinary household cable is weak so the probe will need to be very close to the cable. With some energy monitors, such as the wattson, the sensor/probe clamps tightly around one of the power cables near your fusebox.)
  6. Your energy monitor measures the size of this secondary current and either converts it into a measurement in watts (W) or calculates the running cost per hour.

Using energy monitors to save energy

Some electricity monitors simply tell you the instantaneous cost per hour of whatever appliance you're "probing". Others, like wattson, are more sophisticated: you hook them up to your computer and run a sophisticated statistical program to discover how much energy you're using (and saving) over hours, days, or even weeks. Using a computer program called holmes, wattson can monitor your energy consumption for a 28-day period and claims it could help cut 5-20 percent from your electricity bill. With savings like that, energy monitors can pay for themselves in no time!

Bar chart (histogram) showing which household appliances use most energy in kilowatts (kW)

Photo: You might think you're "saving the planet" by switching to energy-saving lamps, but this chart shows you that's not necessarily true: they use only a fraction of the electricity that other appliances use (particularly the ones that need to generate heat or have big electric motors). At 4000 watts (4 kW), a clothes dryer uses energy 360 times more quickly than an 11-watt (11 W) compact fluorescent lamp (CFL). Lights are usually on for much longer, but even if you use a lamp for 10 hours and a clothes dryer for only an hour, the dryer still uses 36 times more energy!

Calculating your energy use the old-fashioned way

General photo of calculator

If you don't have an energy consumption meter, you can still figure out how much an electrical appliance costs to run by getting out your pocket calculator.

Have a look at the appliance (or the instruction book that came with it) to find out its power rating (it'll be labeled as so many watts or W—about 100 watts for a bright, old-fashioned lamp or maybe 1000-1500 watts for a vacuum cleaner). This is a measurement of how much energy the appliance uses on average each second, so a 100 watt lamp uses 100 joules of energy each second.

To find out how much that costs, you need to figure out how much energy you'd use if you left the appliance on for an hour. That gives you a measurement in kilowatt hours. (A kilowatt is 1000 watts.) A 100 watt lamp running for 1 hour = 0.1 kilowatts for one hour = 0.1 kilowatt hours. A 1000 watt vacuum operating for 1 hour = 1 kilowatt hour. If you know how much your electricity company charges you per kilowatt hour, in cents (or pence or whatever your currency happens to be), you can find out the cost per hour by multiplying the number of kilowatt hours it uses in one hour by the hourly cost. I pay about 20 cents per kilowatt hour, so a 100-watt lamp costs me 2 cents per hour to run and a 1000 watt vacuum costs me 20 cents per hour.

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Manufacturers and suppliers of energy monitors

If you're looking for an energy monitor, there are lots of different ones on the market. Not all of them are available worldwide, so you'll need to do some research before you buy:

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Text copyright © Chris Woodford 2008. All rights reserved. Full copyright notice and terms of use.

Bye Bye Standby is a registered trademark of Enistic Limited. Kill-A-Watt is a registered trademark of P3 International.

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Woodford, Chris. (2008) Energy Monitors. Retrieved from http://www.explainthatstuff.com/energymonitors.html. [Accessed (Insert date here)]

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