Measure-my-use gadgets

Mains-power meter

The Centameter tells you how much power you're using as a household. It costs $190, although a small discount is available for customers of some power retailers.

It's supplied with a sensor - you need to get an electrician to clip this around the main power feed at the back of your meter board. The sensor connects via a cable to a battery-powered transmitter unit mounted on the wall close to the meter board.

The transmitter sends the amount of electrical current you are using to a battery-powered display unit (pictured right), which can be placed somewhere convenient.

You set up the display unit using the nominal voltage (230 volts) and the cost per unit you pay for electricity. (You can even set the greenhouse-gas emissions per unit of electricity - not very useful here because our electricity is generated from a mix of sources that change with circumstances.)

Usually you'd chose to display either your household power use at any given moment, or its cost per hour.

Finding out what any individual appliance contributes to your electricity load is fiddly, though. First you note your total power reading with the appliance going. You then switch off the appliance and note the new reading. The difference between the two readings is the appliance's power consumption.

Plug-in meter

The Elto power meter ($20 to $25) works a little differently. You plug the Elto into a socket and then plug the appliance into the Elto.

The Elto can be set up to display technical information like voltage, current, frequency, power factor, watts, and kilowatt hours. But for most householders the most useful read-out will be running costs. Key-in the price you pay for electricity and the appliance's running cost in cents per hour is displayed.

Are they any good?

Neither the Centameter nor the Elto power meter are laboratory test instruments and so their accuracy is limited. But they do give a good enough indication of the running costs of your appliances - with one exception: they're not accurate at measuring small amounts of electrical current (less than one amp). So they're not likely to accurately tell you the standby power consumption, but they could be useful for comparisons between appliances.

The Elto is likely to be more accurate than the Centameter if you want to measure the power use of appliances with motors in them - such as heat pumps and washing machines. (Why? See "Pointy-head techie talk" below.)

Pointy-head techie talk

To calculate electrical power accurately you need to measure both the supply voltage and the electrical current the appliance is consuming.

The Centameter measures electrical current but not the voltage. It assumes the mains voltage stays constant at 230 volts. Because of this, it'll overstate the power use of some appliances - especially those that have electric motors (for instance, heat pumps).

The Elto power meter measures both voltage and current. This means the Elto can calculate a thing called the power factor, so is likely to be more accurate for appliances containing motors - and for occasions when the mains voltage supply varies from the nominal 230 volts.