Yes, this works - at low or a lower power. My current oven has settings for 10 different gradations or whatever. But unless I’m being lazy or impatient I find a conventional oven just has better results. Analog versus Digital, man! It’s like, you know Vinyl versus CDs!! /s
I was told here on the SDMB some time ago that microwaves with low power settings exist (as in, instead of 1000W it’ll run at 500W). I have never seen one. My experience (anecdotal) is microwaves run at 100% power all the time. What the power setting does is turn off the magnetron and turns it back on again on a timer. This allows the parts that have been heated time to heat up surrounding food without blasting it non-stop.
Look for one with “inverter technology.”
What’s the difference between turning 1000W on and off again and running at a constant 500W?
What gets me is, I’ve seen popularized science columns explaining microwaves, that very patiently explained that it was absolutely physically impossible to vary the power of a microwave (because waves are either waving, or they’re not).
I think people have said that 500W microwaves exist, which they do.
But you are right. A Magnetron is similar to a TV tube, and requires a fixed amount pf power to operate at a given frequency. In a microwave, the magnetron is a single frequency device, and therefore must operate at a specific voltage. The power setting just changes the duty cycle of the magnetron (ratio of ON to OFF times) which changes the power delivered to the food and slows down the rate of heating.
Defrost and reheat settings usually do that as well, to help the heat circulate naturally so you don’t burn the outside while leaving the inside frozen.
There’s an important distinction that you’re glossing over. A standard microwave changes the duty cycle by pulsing on/off every few seconds. You can actually hear the difference, or see it if you have incandescent lights on the same circuit (they’ll dim a bit when it is on). These aren’t so great since the smoothing time is so long. Some parts may overheat before the cooling cycle begins.
In contrast, “inverter” microwaves pulse at tens of kilohertz. Technically, it’s still just changing the duty cycle, but it’s so rapid that it may as well be continuous. They’re also more efficient since the power supply is more akin to a switched-mode power supply than a linear transformer.
I’d expect higher efficiency than that, since making heat is usually the way energy gets wasted, but with a kettle, making heat is the exact purpose of the thing. Losses still happen - heat is radiated away from the whole setup and some is lost as steam is created, so the phase change is taking some energy as well as the steam just carrying away some of the heat from the water inside.
Based on your numbers, I’m calculating about 1600 W of wall socket power going into your microwave. So if it’s only 40% efficient at heating your water over 9.33 minutes, that’s a lot of heat going elsewhere.
But one thing that distinguishes a microwave from another heating device is that the interior (and exterior, for that matter) does not get that hot, just the stuff being heated. Where did all that waste heat go?
If you don’t have much mass inside the oven chamber to absorb microwave energy (as is the case for a single cup of water), then a lot that energy gets reflected back to the magnetron, where it manifests as heat. Microwave ovens include a cooling fan to carry waste heat away from the magnetron, but it has its limits: the standard advice is to not operate a microwave oven completely empty, lest you overheat and damage the magnetron.
There’s also a voltage doubler in the power supply circuit, one component of which is (at least in older models) a large iron-core transformer that also generates some waste heat.
If you have ever tried “defrosting” meat in a microwave you find the edges get cooked while the inside is still frozen/cold. The high power nukes the edges before the inside can thaw out.
I would think a lower power setting (500W instead of 1000W) would be less likely too cook the hell out of the edges. The on/off thing most microwave ovens do I do not think really allows enough time for the hot outside to conduct heat to the inside. If done in a way that doesn’t nuke the edges of your food it takes so long that you may as well defrost on the counter.
For example:
Imagine putting your hand on a red-hot heating element on your stove (disclaimer: do not try this at home…or anywhere). You will absolutely wreck the skin touching the heating element. But, the skin on the back of your hand is fine.
Now imagine putting your hand on a heating pad. Much lower power, no damage and, with a little time, it will heat the whole hand.
The magnetron inside a microwave is not very efficient - it heats up quite a lot as it operates and has to be cooled by a fan (you can probably hear the fan when you turn on the microwave) - the fan itself consumes some power and also produces some waste heat - this heat is vented into the room - it doesn’t go into the thing being heated inside the cavity.
Thanks (also thank you to @Machine_Elf )
I also see that the process of boiling water will entail some waste, as the water will heat the surrounding air, which is also vented from the box. And if any steam is generated, that too leaves the system in the form of waste heat.
All of this seems to confirm my original assumption that the lady I saw on TV would be better off using an electric kettle, provided that she doesn’t boil more water than she needs.
Thermal runaway is a major cause of uneven defrosting:
Liquid water absorbs microwaves better than ice. So when small parts start to actually defrost they then start to absorb microwaves better than nearby frozen material. In short order, thawed spots become cooked spots while the surrounding material remains utterly frozen.
I had the kill-o-watt out so I decided to measure how much energy it takes to use my electric kettle to boil the water for my morning coffee.
20 ounces of water
67.8°F starting temperature
203.8°F ending temperature
I turned on the kettle and let it run until the automatic shutoff turned it off at a full rolling boil.
Starting power usage was 0 kWh, and the kettle has no vampire drain.
Running the kettle uses 1000 to 1200 watts.
Ending power usage was 0.06kWh.
Based on my time of use rate of $0.11/kWh, the boiling the water cost of my cup of coffee was $0.0066. Even at my highest cost time of day ($0.28/kWh) it would only cost me $0.0168 to boil 20 ounces of water.
So even at the most expensive time of day, the cost to boil water is half the cost of the coffee filter. If I make a cup of coffee every morning for a year it will cost me $2.50 to boil the water. If I get an army with reflecting mirrors to help me boil water for free, I still will not have saved enough money to buy a cup of coffee.
Regarding the talk of microwaves: I find that ironically patience is the most important aspect of cooking or reheating food in a microwave.
As others have said, it takes time for the heat to distribute through the food. Because of that I often have a complicated heating program to warm things up. For example, this morning I had a simple breakfast burrito assembled from separate ingredients (not a frozen burrito).
I program the microwave to do 1:10 at full power, and then another 1:10 at half power. This gets an initial heat load into the burrito, and then continues adding some heat, while allowing time for the heat to transfer to the center. 2 minutes at full power gets the edges much too hot. 1:10 of full and then letting it stand does not get it hot enough, so the double cook arrangement is best.
I use a similar full power then low power cycle for reheating most things. The only exception is soup, where full power, stir, full power, stir, works better.
According to a website I found my kettle uses around 0.11kWh to heat 1 litre of water, which, if my arithmetic is correct, converts to 6.51kWh for 20 oz - near enough the same as echoreply above.
However, in England that 6.52kWh would @ 31.24p per kWh, cost me a little over 0.20p. One a day would be 74.34p for a year.
In practice, I almost never use the m’wave to warm up solids. Soup, cold coffee; even frozen bolognese sauce and curry work well. I prefer to let frozen chicken and steak thaw out overnight at 4C in the fridge.
I have tried that and it seems to take a lot longer than a day for meat to completely thaw in my fridge (which is close to your temperature…give or take a degree or two).
Which is fine…but the pound of ground beef I put in the fridge before bed is definitely not thawed by dinner time the next day. Not even particularly close.
Maybe physics is different in my fridge.
The liter to ounce conversion is about right, because I fill to the 0.5 liter line, and then just a bit more to the top of “0.5” mark above the line.
I think you’ve missed a couple of decimal points, though, as that should be 0.0652kWh, which means £0.02 per day, not £0.20 per day and £7.43 per year. An easy mistake, and one I always assume I’ll make. (6.52kWh is enough energy to drive my car 25 miles!)