Do microwave ovens save energy?

I used to think that cooking with a microwave uses less energy than cooking on a hob but in practice this is not always the case - it depends on what you are cooking. In comparison to a gas hob, the microwave oven can also cost more and generate more carbon emissions even when using the same amount of energy, because gas is cheaper than electricity and is less carbon intensive. Of course the microwave can be more convenient, even when it is less efficient. I've been investigating which jobs the microwave is good for and why it isn't as efficient as you might think.

Microwave ovens are supposed to be efficient because they heat food directly. This is because the oven generates microwaves which interact specifically with the food, especially water, exciting the molecules which raises the temperature. However, although the microwave heating mechanism is very efficient the process of generating the microwaves is less so - probably only about 65% of the electric energy used by the oven is converted to microwave energy and heats the food (1). This is staggeringly more efficient than a conventional oven, which has an efficiency around 10% (2). However, when the microwave competes with cooking on the hob the result is less clear cut. A gas hob is typically about 33% efficient but an induction hob can be up to 90% efficient. So the induction hob beats the microwave on energy use and the gas hob wins on cost and carbon emissions, other things being equal. However, you also need to take into account what you are heating. In 2006 DEFRA commissioned some tests to determine how much energy microwave ovens could save, compared to an electric oven or hob, and found huge variation between different tasks (3). The following table summarises my findings which I discuss in more detail below.

	Microwave	Hob
Defrost	**	*	The microwave is convenient but with a bit of forethought you can do this with no energy at all.
Heating Liquids	*	***	Heating up liquids in a saucepan is usually more energy efficient except for very small amounts.
Heating Solids	***	*	Microwaves are very good for heating solids which contain some water.
Cooking vegetables	***	*	Boiling vegetables in water takes up to 4 times as much energy as cooking in a microwave with a minimum of water.

Defrosting foods

Microwaves are not especially good for defrosting foods, but neither is the hob. The trouble with the microwave is that it works much better with water than with ice and this leads to uneven melting. For something like sausages, the bits that get defrosted first rapidly start to get hot and cook while other parts are still frozen. This is why microwaves have an on/off defrost cycle; the rests between heating are supposed to allow time for temperatures to even out.

You can't defrost sausages on a hob at all but you can melt, for example, frozen blocks of stock. The trouble there is that the ice makes bad contact with the bottom of the pan and the first few tablespoons of melt can easily boil away if you aren't careful. Once you have a good pool of liquid the saucepan method is quite efficient, however.

The DEFRA study didn't try using a hob for defrosting, though it did find that the microwave was much more efficient than an oven for heating frozen ready meals or cooking frozen pizza.

The most energy efficient way to defrost things is to put them in the fridge. It takes longer but it saves energy because the food defrosting sucks heat out of the fridge and helps to keep it cool. Obviously this does require forward planning. You can defrost things more quickly without using heating either by submerging the item in a bowl of water (assuming the food container is watertight) or by sitting them on an aluminium baking tray. In both cases the key is to conduct heat into the food as fast as possible.

Heating up liquids

Suppose you want to reheat some baked beans or some soup. The microwave is very good at this because the food contains a lot of water and  absorbs the microwave energy very well. The hob approach suffers because you need to warm up the saucepan as well as the food. However, in my experiments (reported in my book) with boiling potatoes the saucepan only used 15% of the energy. The DEFRA study found that when heating up a single portion of baked beans the electric hob (the ordinary type, not an induction hob) used about the same energy as the microwave and for two portions (so the saucepan heat capacity is proportionately less)  the hob used a third less energy than the microwave. The microwave was not as efficient as it could have been because it heated the beans too much.  When you are using a saucepan it is much easier to stop as soon as the food is warm enough.

 

Porridge

Cooking porridge is a combination of heating the liquid and cooking the oats. Initially the oats contain very little water and they warm up quite slowly in comparison to the liquid which tends to froth over. The DEFRA study found that cooking porridge in a saucepan was more energy efficient than the microwave except for small quantities.

Heating up solids

Suppose you want to reheat some rice. You can do this on a hob by steaming but that involves heating up quite a lot of water and turning it to steam which takes a lot of energy. The microwave heats the rice very well because there is sufficient water in the rice to absorb the microwave energy effectively.

Cooking vegetables

The DEFRA study found that boiling new potatoes in a saucepan took 3-4 times as much energy as cooking in the microwave and the numbers were similar for cooking frozen vegetables and for cooking salmon fillet (compared to poaching in a saucepan). In all these cases the microwave wins because you only have to add a little bit of water whereas when cooking in a saucepan you have to use enough to cover the food. In my cooking experiments 40% of the energy went into water and another 19% into steam (even though I did keep the lid on).

However, to cook in a microwave your vegetables do have to be in small pieces.

 

Conclusion

Since we have a gas hob I shall continue to use the microwave for reheating small amounts of food and the saucepan for most other things. For vegetables the 3-4 times energy hit is more or less offset by the higher carbon emissions of the electricity. If I had an electric hob, even an induction hob, then I would be thinking now about how to make better use of the microwave.

(1) My microwave takes 1200 W to generate 800W. Wikipedia reckons a typical microwave is 64% efficient. Newer ones may be better than this.

(2) Electric ovens are around 12% efficient and gas ovens half that, according to a study by Lawrence Berkeley National Laboratory for the US DOE. This dates from 1996 and could be out of date. However, the numbers are broadly consistent with my cooking experiments which I report in my book.

(3) BNCK07: Comparing energy use in microwave ovens with traditional electric fuelled methods (2006) DEFRA.