# Is a microwave's output power proportional to the mass of its contents?

My friend and I are having a heated debate.

On the one hand, he thinks that a microwave oven that is empty, consumes almost no power (not considering lights, lcd, etc). He says that once you put an item in the oven, such as a glass of water, the oven will begin to consume more power as the magnetron must output more energy to heat the contents. Basically, he says that the power consumption of the magnetron in a microwave oven is directly proportional to the mass of heat-absorbing molecules inside the oven.

On the other hand, there is me. I think that the magnetron is always outputting what it's rated for (in an ideal world). I believe that an empty microwave simply dissipates its energy as heat through the chassis. I made the analogy of a radio tower that when transmitting, is always doing so at the same power regardless of the amount of listeners.

Both of us have come up with some interesting arguments, but neither of us are engineers and lack the knowledge to prove our theories.

So we turn to you!

Thanks!

• We dont have one that can interface between the wall and microwave. Dec 15, 2015 at 17:59
• If you run a microwave empty, you will soon be shopping for a new microwave, in my experience. Lacking anything to absorb the energy it finds somewhere to destructively go...which is why I won't be using my Kill-A-Watt and my microwave to "test" your friend's theory. Grab one at ta thrift store if it keeps you awake at night, but I suggest doing the "with something in it" test first. Dec 15, 2015 at 19:39
• Have you and your friend already tried running a water pump dry? I mean, why it's always the microwave that gets abused? Dec 15, 2015 at 22:50
• @nocomprende You seem so sure of your answer yet others have offered extremely sound and plausible explanations to the contrary. I wonder, do you have any supporting evidence that apply to a microwave? I fail to see what a car engine, using gasoline, has to do with the intricate electromagnetic inner workings of a microwave. Dec 16, 2015 at 0:17
• measurements trump opinions. Do it the scientific way and measure Dec 16, 2015 at 12:02

Simple thought or practical experiment:

If he's right then the heating time to bring water to boiling point is independent of the quantity of water. One cup will take as long as two.

If you're right two cups of water will take twice as long to boil.

• To the break room! Dec 15, 2015 at 17:20
• Hello, this is "the friend" talking now: I think that's a good point, I think one cup will take less time than two... but I think that if you have one cup or two, the microwave will be consuming it's rated power. However, when the microwave is empty, I think that it is not consuming any power (or very little of it) Dec 15, 2015 at 17:28
• A 1200W microwave isn't going to output more than 1200W regardless of the mass of the contents therein. That doesn't mean, however, that a 1200W microwave that was empty except for a 1mm^3 piece of ice (1mg) which would be able to impart 1200W into that ice (which would melt it in 350usec). Increasing the mass of a given material will increase power absorption toward some asymptotic limit. Dec 15, 2015 at 17:39
• We aren't so much curious about how much energy is imparted into the contents, but rather how much is consumed by the oven whether empty or not. Dec 15, 2015 at 17:42
• Two cups would take twice as long to boil as one using an electric heating element as well, but it does not alter the power draw in any way.
– user56384
Dec 16, 2015 at 0:10

Is a microwave oven's output power proportional to the mass of its contents? No. The magnetron develops a certain electromagnetic field strength (volts per millimeter or however it is measured) just like any radio transmitter. I tried the experiment, measuring power draw for 2 cups water, 4 and 8, it was identical: 1380 Watts for a 700 Watt rated unit. This is about what we would expect taking in to account losses (most radio transmitters are about 50% efficient).

When operating a transmitter, there is a specification called standing wave ratio, which determines how well matched the source is to the load. If the load is perfectly matched, it absorbs the entire output, regardless of how much power that might be. If it is poorly matched, some of the power is "reflected back" and makes an in-phase voltage at the output terminal of the device.

If this reflected power causes the maximum safe voltage of the output device to be exceeded, it will arc over. It is also possible for a mismatched load to draw too much current, so the device will self-destruct by overheating.

In essence, you have X amount of watts coming out, which will either be properly absorbed by a load, or will stress the device (magnetron in this case) and probably damage or destroy it. The output power is unchanged, and the input draw is unchanged. It is like connecting an electric motor to a load: Stall the motor and it might burn out, unload it and it might overspeed and damage itself.

This is true for all forms of radio wave emission.

Addition: All devices have loss as well, so even if "spinning the wheels" it will still draw and waste some energy. In the case of a Class A audio amplifier, this is 50% of the input power. In some systems it is more, in others less. Since a magnetron is not ideal, it is simply going to draw some power no matter what.

• Am I to understand that in this case, the load is the content of the microwave? Essentially you are saying that the contents of a microwave need to match, as closely as possible, the magnetron in order for the maximum power to be imparted. So not only would running a microwave empty potentially damage it, but running it with say, a cup of flour, would also damage it as flour has little water (or other substance tuned to a microwave). Dec 16, 2015 at 0:13
• A poorly absorbing load can damage it, yes. Water and oils or fats absorb best. Dry powders very little, although local heating can cause scorching. Two small objects touching (like two meatballs) can cause burning at the point of contact, because each acts like a small antenna collecting power at different phases, so there is a voltage difference. Lots of effects from microwave energy, hard to predict.
– user56384
Dec 16, 2015 at 0:16
• So the real danger of an empty microwave would be from the addition of the original wave and a reflected, in-phase wave causing a voltage over the maximum rated voltage of the magnetron. Very interesting! I assumed that the reflected wave would be out-of-phase with the original wave. Dec 16, 2015 at 0:20
• See: Standing Wave Ratio by the ARRL. I have been an Amateur Radio Operator since 1989.
– user56384
Dec 16, 2015 at 0:34
• You are unquestionably right for waveguides/resonant cavities, but since the microwave oven is mode-stirred, the reflected waves will have random phase and won't change the VSWR. My understanding is that the magnetron is still heated by this reflected power and so fails after a time regardless (e.g., the envelope melts). However, I admit that I'm not sure which is the more important failure mode in practice and few seem to have done the experiment. Most modern magnetrons seem to be fairly robust to both high VSWR and overheating. Dec 16, 2015 at 5:34

So I have finally gotten around to testing this.

Using this watt meter I tested 0, 1, 2 and 4 cups of water in a plastic bowl.

Overall, there was a difference of roughly 1 watt between any of the quantities of water, including empty.

• Good. Now, you'll hopefully get some sleep after all this time.
– dim
Mar 14, 2018 at 20:15

A magnetron that heats food produces a frequency of about 2.5 GHz. An antenna is formed within the oven and as per antenna theories, beyond about one wavelength a true electromagnetic wave is generated and real power is produced. One wavelength is about 120mm. This power is done and gone forever so unless there is some kind of reflection up close to the antenna plate I reckon the power taken by the magnetron will be pretty steady whether there's food inside or not. Don't try it at home.

The stirrer distributes the microwaves to a bigger area inside the oven by reflection. Once they have exited this point there is no coming back for any energy reconciliation.

• I'd like to mark this as the right answer, but reckoning isn't quite going to cut it. Can you point to an article of wikipedia page that supports or proves your claim? Or perhaps a page on allaboutcircuits that explains a principle we are missing? Thanks. Dec 15, 2015 at 18:45
• Thank you for the informative edit. To put my friend's mind at ease, could you explain where the energy produced goes in the case of an empty microwave? I posit that it is absorbed by the chassis and radiated with fans. Dec 15, 2015 at 18:54
• Don't be so hasty to mark it as the correct answer - let others have their say and comment. It's important this question is answered correctly. There maybe something online you can find. Dec 15, 2015 at 18:54
• Yes. The walls absorb some, the glass plate itself absorbs a lot. Some does reflect back into the magnetron, but they're designed to work into a terrible load. Dec 15, 2015 at 21:09

When the oven is turned on, the magnetron emits a transversal elecromagnetic wave into the cavity. As soon the EM wave fills the cavity it begins to hit the walls an reflects back, now the standing wave is formed. In standing wave the E-field and H-field is out of phase by 90 degrees, that means no real power is transfering it just bounces from wall to wall and to the magnetron, where the E-field is exactly in phase and magnitude as the field produced by magnetron, this implies that there is no potential difference and emmiting is blocked.
When there is an item in the cavity, then it bends the standing wave, such that magnetron "sees" the load and additional TEM wave is superimposed to the existing standing wave.
Practicaly the standing wave acts like a conducting channel and "brings" the load connected to the source without wires.

• Please help me make sure I understand. When you say "no real power is transfering", does that mean that a wattmeter between the wall socket and the microwave would read 0? Dec 15, 2015 at 19:13
• @MatthewGoulart Yes, exactly. Of course the walls are not superconducting, so there is a loss, but the magnetron actualy can see the load, if you put a metal ring it will blow the fuse - short circuit and if nothing is inside then it free runs. Dec 15, 2015 at 19:16
• @dbanet I beg your pardon, but I don't understand what are you asking :" So how does that damage the thing and which part of it exactly? " Now what damage and what thing are you refering at? Dec 15, 2015 at 21:01
• Sorry for being not specific enough. Let me rephrase: "how exactly does operating a microwave oven without anything in it damage the microwave oven? Which exact part of it fails in which way?" Dec 15, 2015 at 21:04
• My question was motivated exclusively by the fact this operational mode is highly not recommended by the majority of manufacturers (now referring to user manuals, and by highly not recommended'' I mean they outright say this will likely damage the device) and your claim no actual power-transimitting emission occurs. Dec 15, 2015 at 21:08

A simple way to get the answer is to place a current reader between the plug and your microwave (example: https://en.wikipedia.org/wiki/Kill_A_Watt#/media/File:P3-Kill-a-watt.jpg)

Then run it empty, then with something in it.

• All domestic microwaves use magnetrons which are not efficient .It is the cheap way to get high power and other expensive ways of making 2.4 GHZ are not very efficient anyway.This implies that you wont see big changes on your power meter.Because it is an east test I think you should still do it .+1 Dec 16, 2015 at 19:00
• We are trying to find a similar device and will update the question when we have a definitive answer. Dec 16, 2015 at 21:47
• I have now done that, using a home-made shunt to read watts using my multimeter. It is flat: same draw for 2 cups water, 4 or 8. I did NOT run it empty.
– user56384
Dec 16, 2015 at 22:26
• @nocomprende Would you be able to do the experiment with nothing in the microwave? Dec 17, 2015 at 12:16

The output of the magnetron must be constant for it to work. However, two cups of water take sqrt(2) times as long to boil- square law. This is due to the surface area.

• Please don't use all-capitals. StackExchange has got plenty of other ways to do emphasis. Dec 16, 2015 at 21:08
• Why do you say the output must be constant? How do you reconcile this with what others are saying about the magnetron effectively outputting nothing because of a standing wave that is developed? Dec 17, 2015 at 12:15

I know this question is history now but I would like to confirm that the oven consumes the same power whatever is in it.

Also that my '600 Watt' oven consumes about 1300 Watts when running

• Thank you for answering after all this time. We haven't slept since we asked the question. May I ask what method you used to test this? Apr 25, 2017 at 13:49
• This "answer" needs more depth and/or citations.
– Bort
Mar 8, 2018 at 18:18

The opposite has been confirmed. An empty microwave will consume less power than a full one.

In laymans terms. When the Microwave is empty microwaves are reflected back tot he magnetron blocking it from generating more.

The 1st law of thermodynamics comes to mind.

• If this has been "confirmed" please cite how. An empty microwave has a good chance of breaking, because it's outputting the full amount of power, but that power has nowhere to go. Mar 8, 2018 at 19:39