# How efficient is a microwave oven

1. Oct 23, 2005

### Pengwuino

One thing i was wondering is how efficient is a microwave? I kinda feel like its equivalent to using a firehose on a paper sack fire. So does anyone really know how efficient they are when it comes to heating up your food? And i mean like... efficiency... i mean how much energy do you need to heat up a certain piece of food vs. how much energy is eventually pumped out of the microwave in that amount of time.

Sounds like a good experiment... but im still tryen to figure out what kinda food would work for it...

2. Oct 23, 2005

### Andre

Now let me see. My microwave is 800Watt. It takes about 90 seconds to boil a cup (200 gram) water from 15 degreesC, the normal water temperature in the system.

on the back of the envelope,In the old fashioned way that would be 200 * (100-15) = 17000 calories = 71400 joule is 793.3 joules a second. or lets say a efficiency of roughly 99%

And where else didthe heat go?

3. Oct 23, 2005

### Pengwuino

haha oh yah... water works well... why didn't i think of that.

4. Oct 23, 2005

### Integral

Staff Emeritus
To determine the efficiency you need to look at the power drawn from the wall socket, in comparison to the power put into the water. I think that you will find the overall efficiency to be rather unspectacular.

5. Oct 23, 2005

### Pengwuino

Well I have a meter that can do just that! So really, as far as my experiment goes (and neglecting internal misc. circuitry), i just need to measure the power coming from the wall and what my water does.

6. Oct 23, 2005

### Staff: Mentor

I think you guys are talking about two different things: Penguinman asked about how much of the microwave energy goes into your food, not how efficient the device is at creating the microwaves. So the answer to the question is virtually 100% of the microwaves go into heating your food, since there is nowhere else for them to go.

7. Oct 23, 2005

### Integral

Staff Emeritus
Engineers

The only measure of efficiency that has any meaning is, how much of what I pull out of the wall, goes into the food.

Even correctly considering the efficiency of microwave production, the microwave is better then the stove top at heating a cup of water.

8. Oct 23, 2005

### Pengwuino

Well i was actually originally thinking how much of the energy that leaves the magnetron gets to the food but this is all good stuff too!

9. Nov 22, 2010

### K9EQ

Microwave ovens are not as efficient as one might think. I conducted experiments on two ovens. #1 was built in 1987 and #2 is a built-in from 2000. Input power was net magnetron input measured using a Kill A Watt and subtracting off idle power (lamps, blower, turn table) by measuring power when magnetron was on and off in defrost mode. 500 ml of water in a plastic measuring cup was heated for 122 seconds (2 seconds to allow magnetron to warm up).
#1 rated at 1,200 watts had 1105 watts input and 510 watts delivered to the water for 46% efficiency.
#2 rated at 1,580 watts had 1413 watts input and 740 watts delivered to the water for 52% efficiency.
Other materials will likely not absorb energy as efficiently. But now that my ovens are "calibrated" I can adjust cooking times between them and know when they're going bad.

10. Nov 22, 2010

### G01

Thanks for the input K9EQ, but this thread is 6 years old!

Necroposting clutters the forums and makes it harder to separate new discussions from old ones. Please avoid it.

11. Nov 22, 2010

### K9EQ

But physics doesn't change much in 6 years, particularly where the efficiency of microwave ovens is involved. If you Google "microwave oven efficiency", this is the #4 hit and the first three items don't provide any useful information regarding efficiency - hence the post. Google makes this a current topic, so I would argue that my previous post is an appropriate post.

Thanks for your feedback, but I would humbly suggest your post is off-topic.

12. Nov 22, 2010

### Staff: Mentor

His post is not off-topic, but your point is well taken. We discuss necroposting in the Mentor forums, and the consensus is that it is okay when it adds to the information in the thread, and is still timely.

Welcome to the PF, BTW.

13. Nov 24, 2010

### cdotter

Wouldn't the efficiency depend on the type of food? A more polar molecule will rotate better than a less polar molecule.