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Wouldn't it be more energy efficient and or safe to use microwaves to heat our home's water boiler instead of using dangerous gas or hot electric coils that could catch other things on fire? I'm kinda asking two questions at once, safety and efficiency.

It seems to me that a microwave heats water very quickly and would be a lot safer than using gas or electrical coils because they have to heat a container that then heats the water in a two step process, whereas, microwaves could directly heat water itself. During a usual two step process, you risk the chance of heating unwanted things around it and it'd be easier to confine microwaves, (A simple mesh screen on a microwave oven) than to confine heat. Can anyone shed any information on this?

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My roommate and I tested this once, using a Kill-A-Watt meter, a microwave and an electric kettle. I don't remember the exact numbers, but the kettle, while slower, was a clear winner in efficiency (as expected, since there's not much way for it to waste energy.) The microwave can be expected to lose some efficiency in microwave generation process. – user2963 Mar 9 '12 at 23:35
thought about this 27 years ago tried to have the idea patiented but got told the idea had already been thought of by an american and was not viable what waste of 200 quid. the company was called i c s. based in london. – user9369 May 22 '12 at 21:42
up vote 3 down vote accepted

A microwave oven is only about 65% efficient i.e. 65% of the electricity consumed is converted to microwaves and the rest is dissipated as heat. I suppose you could use the 35% dissipated as heat to heat the water as well, but then why not just use a heating element that dissipates 100% of the electricity as heat?

As for speed, this is normally limited by how much current you can draw from the mains. In this respect there wouldn't be any difference between microwaves and elements, except that unless you can use the waste heat from the microwave magnetron it will take 50% longer than a heating element.

It's not obvious to me that a microwave magnetron is inherently safer than a heating element. If there's a risk from water heating (is there?) it seems to me a better strategy would be to improve the design of heating elements.

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Can't agree more on safety. There are lots of things which might go wrong with high-power magnetrons. – BarsMonster Mar 10 '12 at 10:24

Regarding speed, an immersion heater is a clear winner. Microwave ovens heat small amounts of water quickly, but larger amounts heat slowly. If it takes 2 minutes to boil a cup of water (about 120 ml) in a 1000 watt microwave oven, 2 cups take almost twice that. A gallon of water is almost 4,000 ml - a hot water tank holds 50 gallons and up - so, to heat that in a reasonable amount of time becomes incredibly expensive: you'll need several thousand watts - by the way, you can buy water tank immersion heaters for around $ 20. Also, at 1000 watts you are pushing the limits of a 120 volt circuit, so you'll need a 240 amp circuit as all water heaters do. You might say, let's only heat the water when we need it - it can be done with microwaves but it is incredibly expensive - I analyzed this for a client some years ago. (I'm a microwave scientist and have been doing this for over 50 years)... as to safety what's unsafe about a conventional water heater? A microwave system could be made perfectly safe (after all, our home microwave ovens are totally safe), but when you have several 1000 watts it becomes expensive to provide all the choke systems; further, since there is water involved you need to keep the microwave components separate from the water heater, so you'll need waveguide and a suitable launch & choke. This may be fun to talk about over a beer, but don't invest in it.

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