[Jennifer Wing]

Jennifer Wing  asked the Naked Scientists:
   
Dear whoever ends up reading this,

Mike Berners-Lee in his book "How bad are bananas?" estimates it takes around 18g of CO2 to boil just enough water for a cup of tea or coffee.  Since I do this several times every day, and add to my sins by adding a large slosh of milk, I was wondering what is the most efficient way to boil the water?  Does it actually make a significant difference?  I have the choice between a 1.7L capacity electric kettle, an 850W microwave oven or a 1200W vitro-ceramic stove-top element.

Many thanks in advance for any consideration you may give to this,  

Jenny Wing.

What do you think?

[JP]

I've seen a few discussions on this around the web, but I haven't seen an actual scientific source cited.  Nevertheless, the winner is always the electric kettle, and I believe this.  The stove and microwave have to heat a wide range of things, so they're less efficient at just doing water. 

Here's a link to one discussion on the matter:
http://www.treehugger.com/files/2009/11/electric-kettle-stove-or-microwave.php

[SeanB]

Funny enough the microwave is the best for a single cup. The kettle has a lot of metal in the element ( along with a lot of insulation in the form of powdered silica to insulate the element electrically) and has to heat this up as well as the water and the kettle. You also have to place enough water in to cover the element, and this is typically nearly a second cup of water. You lose energy to heat the cold cup you pour the water into, and you probably have used the amount of energy to heat theoretically 4 cups to do one. The stove is the same, just you have a lot bigger element, a lot of glass or ceramic to heat up as well, and then the pot that holds the water. This makes a good room heater for the 5 minutes or so it takes to heat up.

The microwave is around 50% efficient in converting electricity to heat, but almost all of the energy it places in the cavity is absorbed by the single cup of water inside, nothing else gets hot inside.

If you are doing 15l of water the stove will be best, as you can place a 15l pot on it, whilst the kettle is best for doing 6 cups of hot water.

[Geezer]

The best method would be to use one of those small electric immersion heaters and use it to heat the required amount of water in the cup (well insulated, of course).

[JP]

It just occurred to me, that the microwave/kettle are easy to test at home.  The wattage on both is listed, so you just need to compare the time to bring a cup to a boil...

[syhprum]

Sean B
"The microwave is around 50% efficient in converting electricity to heat", of course the Microwave cooker converts virtually a the electrical energy it takes in into heat (a tiny amount is lost as radiation) I think this should read as useful heat in the object to be heated.

[CZARCAR]

Funny enough the microwave is the best for a single cup. The kettle has a lot of metal in the element ( along with a lot of insulation in the form of powdered silica to insulate the element electrically) and has to heat this up as well as the water and the kettle. You also have to place enough water in to cover the element, and this is typically nearly a second cup of water. You lose energy to heat the cold cup you pour the water into, and you probably have used the amount of energy to heat theoretically 4 cups to do one. The stove is the same, just you have a lot bigger element, a lot of glass or ceramic to heat up as well, and then the pot that holds the water. This makes a good room heater for the 5 minutes or so it takes to heat up.

The microwave is around 50% efficient in converting electricity to heat, but almost all of the energy it places in the cavity is absorbed by the single cup of water inside, nothing else gets hot inside.

If you are doing 15l of water the stove will be best, as you can place a 15l pot on it, whilst the kettle is best for doing 6 cups of hot water.

plastic cup dont get hot, rotating glass plate might...in my MW

[CZARCAR]

The best method would be to use one of those small electric immersion heaters and use it to heat the required amount of water in the cup (well insulated, of course).

ideally avoiding contact with the cup so to minimize heat transfer &might as well stir it while at it,eh?

[Geezer]

Of course, you could always boil it in a vacuum without adding any heat at all, at all  []

[JP]

Well, I just compared my 1500 Watt kettle to mu 800 Watt microwave.  For 1 cup H20 (in a mug in the microwave), they seemed pretty close.  It took 1:15 for the water to boil in the kettle.  After 2:20 in the microwave (an equivalent amount of energy used), the water was at 200^o F (yeah, my sill American kitchen thermometer measures in that scale).  However, part of the water must have been superheated, since it bubbled up when I put the thermometer in.  But when I stirred it, it dropped to 200 F. 

You can do the experiment yourself, too.  Just time how long the microwave takes to boil water and compare that to how long the kettle does.  Multiply each time in seconds by the wattage rating of the device and you'll have a measure of the energy used.  Whichever is lower wins.

But really--the energy saved is miniscule.  If you're concerned about energy savings, look elsewhere--heating/cooling, hot water, and other major appliances.  I'd probably always choose the kettle, just for the convenience of speed.