Inflation being what it is, there are regular reports on TV of people’s efforts to save energy. A recent one featured a lady who had stopped using her electric kettle and now boils water for tea in her microwave.
At first sight it was an obvious saving, since she only boils enough water for a single mug of tea, but as I boiled our electric kettle for tea this morning it occurred to me that this might be an incorrect assumption. We are pretty good at judging how much water to put in the kettle, so very little gets wasted (but there is some). On the other hand, a very high proportion of the energy that goes into the kettle is converted into boiling water, while the microwave scatters energy all over the inside of the oven.
I, of course, thought that there would be someone on this board with the skills to calculate which is the most efficient in terms of kWh of energy. I don’t know if it helps, but my kettle is 2kW and the microwave 900W.
I don’t know if a kettle or microwave is the most efficient, but I can’t imagine it having any measurable difference on the power bill. People have no sense of perspective and try to save money on the strangest things.
When it comes to electricity, I think it is because it’s so abstract for people.
Note that I boiled 18 oz (550 ml) of water in the kettle, because that’s the minimum safe amount. The mug in the microwave had 14 oz (420 ml)
As I understand it, almost all the energy produced in a microwave ends up in the water. The waves bounce around until they are absorbed by SOMETHING and the water is the predominant medium that absorbs microwaves in the space.
Interesting question. A microwave oven draws some serious current as well, it may not represent a “dead short” or peak draw the way resistance heating elements do, but I suspect the difference in energy use overall is negligible.
A microwave oven is just a tongue-burning rubber food generator. I like them, a lot, for heating water, but that’s about it.
Jonas Ketterle, '08, MS '10, a mechanical engineering graduate who recently won a $75,000 grant from the EPA to help design greener dorms, explains that it all comes down to energy loss during the generation, transmission and final use of electricity. “Say the power plant is burning gas to make electricity,” he says. “Then only 30 to 40 percent of the energy content of that gas arrives at your house.” You’re better off burning the gas directly to heat the water.
If electricity is your only choice, an electric kettle is more efficient than the stove, because the heating element is in direct contact with the water. And microwaves are the least efficient method of all. - Source
I recently did a few experiments on this, but not sure it’s OK to link my own video.
Microwave was terrible, due to the inefficiencies of the emitter (any heating device that also needs a fan to cool it while it’s working cannot be especially efficient)
For me, the electric kettle was about a penny cheaper per litre than boiling the water in a pan over a gas flame, but gas is less efficient in terms of the amount of energy that makes it into the water.
Gas is just cheaper per kwh for me at billing, so it costs less to boil over gas than in a kettle (not sufficiently less to make me want to do it that way).
An electric kettle with submerged element is, in practice, probably about the most efficient way to boil water using electricity.
In theory, it would be still more efficient to boil water in my electric pressure cooker, because it’s very well insulated, but that would be hugely impractical for making a cup of tea.
You can reasonably assume that power consumption in each method scales directly with the quantity of water boiled. At least across a reasonable range of quantities. So to get like-for-like measurements, I’d multiply the microwave result by 18/14 = 1.28 to convert it to the amount needed for an 18oz boil.
Which raises the microwave case to 0.043 * 1.28 = 0.055 kWh. And further erodes the difference in electricity and in money.
In general I agree 100% about the rubber food part. It’s total yuck for many things, and totally OK for rather few things.
As to tongue-burning, try running it at half power for twice as long or 1/3rd power for 3x as long. The food will be much more evenly heated and takes zero additional effort. And is still quicker than heating an oven or stove & warming your [whatever] that way.
This. Judging from @Mighty_Mouse’s numbers, the difference is on the order of 1.3 cents per cup. Two cups a day for a month means you might save as much as $0.78 on your electricity bill if you choose the most efficient heating method. That assumes the high electricity rates of New England. Here in Michigan, the rates are about half of that, so figure $0.39 per month.
There are many other devices in your house that use less power, but spend more time turned on. In my own house, I have a TiVo that draws 37 watts 24/7. At Michigan’s rates, that’s about $3.78 per month. The 1500-watt space heater my wife and I use during showers in the wintertime eats up about $6.39 per month. the 200-watt floor lamp we light up our main room with in the evenings eats up about $1.43 a month. If I were looking to save money, there are a lot of things I’d probably focus on before trying to find the most efficient way to heat a cup of water.
But the point of using the microwave is that you don’t have to heat any more water than you actually use.
Anyway, my guess is that the particulars will vary based on your microwave, your electric kettle, and how much hot water you typically need. And that in almost all cases, this isn’t really worth worrying about.
Agreed. But as the OP said, he’s good at putting only the needed amount of water in his kettle too.
Now of course as @Mighty_Mouse pointed out, there is some minimum safe amount of water in a kettle. Which for single people making single cups may mean heating more than necessary. Unless one has a mini- single serve kettle. Or is making tea for two ( and two for tea ).
OTOH, many electric kettles are also well-insulated, so you can heat, say, 3 cups of water once then use them to make 3 cups of tea over the space of a morning. My late first wife liked tea during the day and did just this.
Alec at Technology Connections on YouTube compared kettles vs. stovetops, gas and electric, and found that the kettle is most efficient and quickest. I haven’t watched the video recently, but I don’t think he even considers microwaves.
(Are we going to be able to embed videos again soon?)
Don’t even get me started on the whole brouhaha about “vampire” wall warts. The scare blurbs always talk about how much power is wasted across the entire US per decade, or whatever, and never mention that it’s a few cents per person per year. It’s almost certainly more cost to take the effort to unplug them.
and two for tea 
).