Electrical energy monitors
by Chris Woodford. Last updated: May 15, 2014.
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
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.
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.
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.