Oven versus microwave

G Magazine

When it comes to heating or preparing a meal, which oven is the greener option?

Microwave versus oven

Credit: iStockphoto

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The slow food movement, the trend to grow your own food – even the prevalence of cooking programs on our televisions – suggests we may see a reversal of recent take-away trends and a move back to the kitchen as a new generation of eco-conscious home cooks prepare more and more meals at home.

In recent decades, household cooking energy use in Australia has declined because of pre-prepared and take-away food and the significant rise in the frequency of dining out.

An estimated four per cent of Australian residential energy is used in cooking. But the way food is purchased and prepared involves energy trade-offs.

While the centralised cooking of pre-prepared or restaurant-served foods reduces cooking and refrigeration energy use, it increases transport energy use as people travel to restaurants or take-away outlets, not to mention all the packaging waste that goes with it.

Home cooking is generally healthier – but how can you make sure it is also more environment-friendly?

Debate about the safety of microwave ovens has raged for years, with advocates championing the energy efficiency of the appliances, while detractors raise the alarm about potential health risks. Most Australian homes have both a conventional electric or gas oven and a microwave oven.

So when it comes to heating food, which is better for the environment – the conventional oven, or the microwave?

Materials and production

Most of the components of a microwave oven are metallic. Inside a wrap-around metallic (or occasionally plastic) case are panels of enamel-coated steel, a ceramic or glass cooking surface and a glass door with a perforated metal plate that reflects microwaves but allows visibility.

The parts you don’t see include electrical and mechanical components like timer motors, switches and relays, a magnetron tube, and various metal and plastic bits. Most microwave parts can be recycled, with electronic components a small part of the overall product.

Electric ovens have a similar construction, only without components such as magnetrons. Most have electronic controls and the majority of components are made from recyclable metals.

For both microwave and electric ovens, safety is a big component of manufacture, with strong quality control procedures in place to ensure the final product doesn’t leak very hot air or allow microwave emission leakage.

Energy use and efficiency

Most microwave ovens generate electromagnetic waves in the frequency of 2.5 gigahertz to heat food. Water molecules in food absorb the microwaves in this frequency, and the heat is then transferred to other molecules and, sometimes, the container holding the food (depending on the material it is made from).

This is why you need to add water to dehydrated foods such as rice and pasta to cook them in a microwave.
Ovens, by comparison, cook food by maintaining a high air temperature throughout the oven cavity, and are usually pre-heated before use.

Nearly all ovens have a heat-retaining glass window on the door to minimise heat loss but each time an oven is opened it loses around 4°C of heat.

The upshot is that microwaves direct around 60 per cent of the energy they use toward cooking food – which is highly efficient compared to the estimated 12 to 14 per cent of all the energy drawn by a standard electric oven.

What about combination microwave ovens, which offer convection cooking and a grill in addition to the usual microwave function? A British study showed that using a combination microwave oven on a convection setting to roast a chicken used 30 per cent less energy than an electric oven.

As with microwaves, most electric ovens now also sport LED displays which use standby electricity all the time.

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