Wave off

by Simon Handby in At home on 07.03.08

We’ve taken a break from testing how much electricity washing machines use to answer a question about microwaves.

Sandra Storr asked us to look at the one-time ovens of the future, as she’d heard that they used as much power on standby as they did during cooking. We’re relieved to find out that’s not the case.

We tested a 10-year-old Sharp microwave with a group E cooking power rating. Despite its age, we found that it only used one Watt when totally idle, which isn’t too bad. Like most microwaves, opening the door turns on the oven light, which added another 21 Watts, so it’s a good idea to make sure it stays closed when not in use.

Half-baked

Things got a little more interesting once we started cooking, though. Our microwave’s 850-Watt rating refers to the amount of microwave energy it can produce for cooking, rather than how much power it requires during the process. We found that it drew almost twice as much power – 1.6 kilowatts (kW) – when running at its maximum setting. Wikipedia says that a modern microwave oven should convert around 64% of the energy it consumes into microwaves, but ours manages only 53%.

Even so, using our microwave for some tasks, such as baking a single potato or reheating chilled food, is still likely to be more efficient than using a conventional oven. A typical electric oven draws 3kW or more for several minutes as it heats up, and does so again periodically to stay hot. That’s even more than a kettle, which is why most ovens are wired directly into a heavy-duty circuit in your home – unfortunately, that’s also why we can’t test them with our power meter.

On the other hand, the way that microwaves heat food means that bigger servings require proportionately longer cooking. This is less pronounced in a conventional oven, so it’s likely to be more efficient if you’re cooking many potatoes. Of course, it’s also a better bet if you prefer them with a crispy skin.

Hot potato

Returning to Sandra’s question, although her microwave isn’t using as much power on standby as she feared, her habit of turning it off at the plug is still saving her electricity and money. In a week of heating up porridge and defrosting the odd frozen meal – wasteful compared to defrosting in the fridge overnight – our microwave used exactly 2 kilowatt hours, costing around 22p.

IMAGE Weiser Farm Potatoes by Flickr user ExperienceLA