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"You’re destroying the planet, Daddy,” my seven-year-old daughter shouted as she stared into the display unit of my new smart meters. It had seemed an ordinary Sunday morning until then.

The tumble dryer was whirring downstairs, the kettle was boiling for my second mug of tea, and a few lights were on here and there. The meters, however, told the true story. Our household was generating the equivalent of 40 tonnes of CO2 a year – from electricity alone. I had no idea how much more was coming from the gas-fired central heating.

They also told me that, right then, I was spending more than 50p an hour on power. And that’s before the implementation of the recently announced rises in power prices that could push bills up by 17%. As Eliza and her brother Tim, 12, rushed around flicking switches off, that amount suddenly went down.

By the time they had finished, we were consuming just 1.2p per hour. Electricity meters have long been unloved and misunderstood devices. We may spend an average of £453 a year per household on power, and rising, but the meters that oversee how and when we spend it remain among the most mysterious of household gadgets.

Their spinning dials and displays may as well be written in Chinese. If you think about it, that’s rather bizarre. How would you feel if your favourite shops stripped all the prices off their goods and instead made you wait months for your bill – then demanded you pay without complaint? If you were shopping, you’d go elsewhere – but, even though you can switch energy supplier these days, you are far less likely to do so.

What’s more, it has been very much in the utility companies’ interest that we should have no idea how much we are consuming. My electricity and gas meters are hidden away in a cupboard in an old coal cellar, and I had no idea what impact switching on the tumble dryer was having on my bills.

The new power-display units are, however, bringing electricity metering out of the closet. There’s a raft of them on the market, but all aim to turn energy-company gobbledegook into easy-to-understand figures. How much are you spending per hour? Per day? Or in a whole year? And how about carbon dioxide?

Working out how much CO2 is produced by generating a given amount of electricity is easy. The energy companies could have printed it on our bills for years, but have refused. Display meters give a way around such environmentally unfriendly obstinacy.

So, how do they work? All of them use a transmitter that clips to the incoming power cables to measure how much energy enters a home. The data is then transmitted to a display unit that can calculate the overall energy consumed, its cost and the amount of CO2 generated.

There the similarities end, however. The parts of these devices that matter to consumers are the display units themselves – the bits that tell you how you are doing – and they vary a lot.

THE OWL
£49.95; www.theowl.com

A bizarre name for a brilliantly straightforward device. The Owl’s display unit is about 4in square, with a nice large screen that can be read from several feet away. This is important. What you want to do with these gadgets is plonk them somewhere that lets you check the figures with a casual glance, perhaps when walking by.

The mode button lets you flick from checking your cost per hour to seeing your CO2 output in tonnes a year – which seemed ridiculous until my children discovered it. That figure made much more sense to them than the cost or the kilowatts.

EFERGY
£44.95; www.efergy.com

Superficially, this is quite similar to the Owl – a white box with a display screen, smaller than its rivals, and some buttons. The Efergy is, however, a very different device. You can scroll back through past days, weeks or months to see if your energy consumption has risen or fallen, or you can view your total consumption since the last time it was reset. This is a grown-up device for someone who wants to track their energy usage and keep records.

WATTSON
£149.50; www.diykyoto.com

The iPod of display meters. The Wattson tells you how much energy you are consuming at a given moment and turns this into an annualised cost. This gave me the startling information that my washing machine, tumble dryer and electric oven, when operating at once, were temporarily using the equivalent of £3,500 worth of power a year. The Wattson even changes colour, glowing a restful blue when you’re using just a little electricity, but turning red when you’re consuming a lot. It also has an electronic memory, so you can save your consumption records to a home computer, using software downloaded from the DIY Kyoto website.

THE VERDICT

The devices will appeal to different markets, but if you have children and want to make them think more about saving energy, then the Wattson and the Owl have the simplest displays. If you want detailed information, the Efergy is the cheaper option, but the Wattson will do that job too.

There are several studies showing that display units can cut average energy bills, and hence emissions, by about 12%. For the average small family home, this means it would take about three years to recover the outlay on the Wattson – compared with one year for the other gadgets. What I found really illuminating was being able to measure the impact of everyday appliances on our bills. Switching on the kettle, for example, sent consumption up by 35p an hour. Fortunately, kettles boil quickly, but the tumble dryer added 23p an hour – and that gets used for several hours a week.

Our kitchen spotlights were another shock. The nine 35-watt halogen bulbs consumed about 4p an hour – seemingly a small amount, but they are in almost constant use through the winter. When you remember that each penny per hour translates into an extra £87 a year on the bill, it makes you keener to switch things off.

The downside – depending on my mood – was that the Owl turned the children into a kind of eco-police force. “You’re committing crimes against the planet,” Tim shouted when I left a computer on one evening. The solution, however, was simple: put the display unit on a high shelf whenever I’d had enough of green guilt trips. Or threaten to turn off the kids’ computer games – all in the fight against global warming, of course.

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