How does the power setting on a microwave work?

In general, how does the microwave power setting work? Is it a straight percentage of the maximum wattage or is it something less precise?

For example, would the time to cook something in a 700 watt microwave be the same as a 1000 watt at .7 power?

Is there any other major deciding factor in how one microwave cooks compared to another besides the power? (Assuming it is actually running at its listed power. Is it even possible for a microwave to drift off its posted power due to age or some other factor?)

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It may already be an implicit assumption, but I think that time to cook is probably not as important as how to cook. While the power setting does affect the time to cook, the importance of time is to control how hot the parts of the food get.

As with all cooking, there is a need to control temperature. Running the microwave at less than full power allows time for the heat to be conducted from the hot parts to the cool parts. It is similar to searing vs. sauteeing.

I think that the accuracy of most microwave oven's power settings are good enough (my own testing shows about 10%). I would be more concerned about how I wanted the item cooked. Some examples:

• When I want to reheat things like a thick soup, I run the microwave at full power for a short period of time, stir, and run the microwave again. Stirring the concoction distributes the heat faster than the conduction process.
• When I want to defrost frozen foods (think meat), I use the lowest power setting. This prevents the edges of the food from getting cooked.
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The majority of microwaves cannot modulate their power output. The power setting in most microwaves simply turns the magnetron (microwave generator) off and in in cycles. So a power setting of .5 for 10 minutes would simply cycle the magnetron on and off every few seconds, with a total on time of 5 minutes. You can actually hear this occurring.

According to wikipedia, some newer microwaves can actually achieve a more or less constant level of reduced power using a technique called pulse-width modulation. I have never seen or used one of these though.

Microwave cooking is never very exact, so cooking something at 700 W vs 1000 W at .7 power would yield very different results. The only reliable way I've found in cooking things properly in different wattage microwave ovens is simple observation.

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+1. Microwaves don't seem linear on wattage. – papin Sep 17 '10 at 19:36
I did some testing on my GE 1000W microwave at a 50% power rating. It seems to cycle at a 30 second period... ~15 seconds on at full power, ~15 seconds off. I was quite surprised to find that the timings drifted by about 10%. Difficult to describe, but after 4 minutes, the cycles were off by about 3 seconds. Probably not really significant for general cooking techiques. – erichui Sep 18 '10 at 1:46
I agree. You can distinctly hear the power shifts in my microwave. If you put it on 5 (out of 10), you get 100% power for 50% of the time. – Dinah Sep 18 '10 at 3:45
@erichui Now I'm curious: What happens if you put it on 50% power for 10 seconds? – Yamikuronue Oct 24 '12 at 16:05
@Yamikuronue I just did you test, 50% power for 10 seconds. The microwave was turned on for the full 10 seconds. – erichui Oct 25 '12 at 0:38