Home energy monitors

by Chris Woodford. Last updated: May 15, 2024.

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 smart meter for checking home energy consumption. Unlike older meters, this one automatically sends its readings to the utility company. You can see from the LCD display that it's currently reading 1446 kWh (kilowatt hours)—in other words, about 1.5 megawatt hours of energy (5400 megajoules) has been used since the meter was installed.

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 slowly 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!

How can you measure electricity consumption?

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.

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.

Charts: More and more of the energy we use at home is going to power gadgets and appliances. Look at the rings of this donut chart, which compares US home energy use from (center ring to outer ring) 1978, 2009, 2015, and 2020 (outer ring). Blue = home heating, Orange = appliances and electronics, Yellow = water heating, Green = air conditioning. You can see that while we're using much less energy to heat our homes, the share of energy use for appliances (the orange segments) has grown enormously, and air conditioning is a real cause for concern. Data from Residential Energy Consumption Surveys (RECS), US Energy Information Administration (EIA), and is the newest available as of May 2024. Nevertheless, an analysis published by the EIA in 2015 shows that efficiency gains of various kinds still led to an overall reduction in energy use per household in the three decades between 1980 and 2009.

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...

How we can measure electricity using magnetism

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.

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 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.

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.

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Figuring out which home appliances cost most to run

Even though most of us can't manage without things like vacuum cleaners and clothes washing machines, there's still scope for saving energy by buying the most efficient appliances we can find. Many countries now have eco-labelling schemes that compel manufacturers to show how efficient (or otherwise) their products are; by encouraging healthy competition, schemes like this lead to a gradual improvement in the efficiency of appliances in the long term. In Europe, appliances are now widely rated on a scale from A to G, with A being most efficient and G most wasteful.

Photo: The packaging for an energy-saving fluorescent lamp shows it has an energy rating of A—it's currently one of the most efficient lamps you can get. Look out for energy ratings like this on all the major appliances you buy, especially on energy guzzlers like clothes dryers, clothes washing machines, and dishwashers.

To make the biggest possible difference to your energy bills, it's important to focus on appliances that use the most energy, such as clothes tumble dryers. Get into the habit of drying your clothes outdoors (often possible even on cold days in winter, providing the humidity is low) and you'll make big savings. If you can get your clothes 50 percent dry outdoors and just finish them off in your dryer, you're still going to save half your drying costs. Dishwashers and clothes washers also use a lot of energy so look into whether you can run them on shorter cycles or lower-temperature programs. Can you run them with bigger loads without sacrificing their cleaning performance? Can you run them safely overnight when electricity is often cheaper? (Make sure you have smoke alarms fitted if you do this.)

Which appliances cost most

Photo: Kettles are among the biggest energy hogs because they draw a lot of power and we use them very often. They're also very efficient at converting electricity into heat, so the only real way to economize is to avoid boiling more water than you need.

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!

To understand how much a particular appliance costs to run, you need to know both how much power it consumes and how long it's working for, which will tell you how much energy it uses in total. Things that use a lot of power only cost a lot of money if they run for long enough to use a significant amount of energy. 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!

Photo: Power hungry? This chart shows the power consumption (in watts, red) and energy used per week (in kilowatt hours, green) for a variety of everyday appliances. You can see that a tumble dryer uses lots of power and energy because it runs for a long time. Things like refrigerators and TVs don't use much power but they consume plenty of energy because they run for a long time each week. Vacuum cleaners use high power but relatively little energy because they run for only a few minutes at a time. Kettles cost more to run than you might think because you use them so often. Energy-saving lights use trivial amounts of power and energy, but you might have 10 of them burning away at a time, so the cost quickly mounts up.

Analyzing the data

Artwork: A typical smartphone energy tracking app. This one is Efergy Engage, which I downloaded from the iPhone App Store.

Most energy monitors let you track, analyze, and chart your consumption over a period of time using PC programs or (often more conveniently) smartphone and tablet apps. If you go into your favorite app store and search for "energy monitor," you'll find quite a few to choose from. Most require you to have bought a bit of energy-monitoring kit in the first place, though there are a few apps that work purely from meter readings (helping to monitor your overall consumption rather than focusing on specific appliances).

Even without an energy monitor, you can make quite a big difference to your electricity and gas consumption simply by getting into the habit of taking regular meter readings, entering them into a spreadsheet, and automatically plotting a chart; the very act of measuring and monitoring has an amazing way of encouraging efficiency—and it doesn't cost you a penny!

Artwork: Making regular measurements of electricity or natural gas meter readings will encourage you to be more efficient, even if you don't use an energy monitor. This is how I cut my gas consumption significantly, over a period of five years, after I started making quarterly meter readings and entering them in a spreadsheet. It's best to make your own readings on consistent dates rather than rely on utility company data, which might be taken on different dates (making it harder to compare the figures from one year to the next).

Buying smart

The trouble with everything I've said so far is that energy consumption is often a "done deal": there's not much you can do about a fairly new refrigerator that uses lots of energy if you've bought it only very recently and don't plan to replace it. (You might be able to turn down the thermostat, but that's about it). You can (and should) factor power consumption into your decision when you buy new things: more efficient appliances (and things like energy-saving lamps) may well pay for themselves within their lifetime. Having said that, it helps to know how things work so you're not compromising on performance, if that's important. Modern vacuum cleaners, for example, often use much more power than older ones because they have more elaborate filtration and it takes more energy to pull air through them. If air purity is really important to you (perhaps because you have asthma), compromising on a lower-power, more energy-efficient cleaner may not be something you want to do if household dust then becomes a problem. But maybe you could investigate making other appliances more energy efficient instead?

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