# How efficient is a microwave oven

1. Oct 30, 2012

### Homer D.

This site was at the top of the Google results for "How efficient is a microwave oven"

I can't restart the old discussion, so I'll start a new thread.

Previous posters came up with numbers from 99% to 46%.
"A microwave oven converts only part of its electrical input into microwave energy. A typical consumer microwave oven consumes 1100 W of electricity in producing 700 W of microwave power, an efficiency of 64%. The other 400 W are dissipated as heat, mostly in the magnetron tube. Additional power is used to operate the lamps, AC power transformer, magnetron cooling fan, food turntable motor and the control circuits. Such wasted heat, along with heat from the product being microwaved, is exhausted as warm air through cooling vents."

Also of course, some energy exits into the room through the door's grid glass. It's not a perfect barrier, but a convenient compromise. Commercial food service units have no glass.

A member "cdotter" offered the observation "Wouldn't the efficiency depend on the type of food? A more polar molecule will rotate better than a less polar molecule."
I agree that efficiency also depends on what's in the oven. Presumably if running it empty the oven would still consume house current and yet nothing's on the turntable to get hot.

I'm wondering if there is any data about this. 99% 64% 46% is such a wide range.

2. Oct 30, 2012

### Staff: Mentor

I think based on your analysis, a wide range is appropriate, no?

One could relatively easily test this by timing how long it takes to boil some water though...

3. Oct 30, 2012

### Staff: Mentor

64% looks reasonable for an efficiency electric -> microwaves (depends on the model, too, of course). The part of the microwaves which gets absorbed by the food depends on the type of food, its quantity and distribution and the microwave, so the total efficiency will be a bit lower and variable.

4. Oct 30, 2012

### nasu

The efficiency quoted (64%) refers to the conversion of electric power to microwaves.
How the microwaves are used is a different story.

5. Oct 30, 2012

### Homer D.

What actually puzzles me most is the contention that when you double the quantity you must double the time to cook. If a given sausage is cooked in one minute and you want two, it seems they are done (split from steam) before the full two minutes are up.

termed "reciprocity failure"

Last edited: Oct 30, 2012
6. Oct 30, 2012

### sophiecentaur

That rule of thumb could apply for large items but the poor matching of the energy into some small items could compromise the actual Mains to RF efficiency.