If the pan is larger than the burner then the edges could be "cool". As I remember, my Fagor guide recommended no more than an inch larger than the burner diameter (e.g., 10" pan on 9" burner). I'll have to recheck tomorrow. See also the cookware link below.

If the pan is smaller than the burner ... "will the reduction in power be noticeable?" You mean that your electrical bill will be lower? That a pan-sensing feature will decrease the burner output (so that a 10 setting is effectively, say, a 7 setting)? That there will be an electromagnetic field beyond the pan edge that is wasted (note that there won't be any reduction in cooking power)? Something else?

Here is a link that might be useful: cookware: diameter extension ...

Sorry I wasn't clear on that. I'm concerned that the pan-sensing feature will decrease the burner output, so that a 10 setting is effectively a 7 setting, for instance. My pans are more likely going to be smaller than the burner, not larger.

On mine (Kenmore) if the bottom of the pot is smaller than the circle designating the desired pot size, the cooktop will simply flash "F" for fault, and the pot won't heat at all.

Most of the burners have two concentric circles. The smaller indicates the minimum pot size, and the larger indicates the maximum burner extent (meaning that any part of the part the extends past this larger circle will only heat through conduction from the rest of the pan.)

This characteristic has been somewhat of a problem for me in that the smallest burner (6") which is the only one I can use for a small sauce pan, is in the back of the cooktop, and is hard to reach if the burners in front of it are in use.

I've used a 6" frying pan on my biggest burner with no apparent reduction in power. It seems as though the pan gets the same amount of power per square inch as a larger pan would.

Looks like it varies depending on model so you may want to test whatever you're thinking of buying with your own pans and a bag of chopped onions.

My cooktop is no longer available and probably less sophisticated than modern ones. Good thing maybe!

I have a Kenmore 30" cooktop. I have never had any problem with pan size. I just went an put a small pan(6") on all the burners and was able to boil water on all burners. No fault warnings at all. I was pretty sure that I had done so in the past when other burners were busy, but checked to make sure. If the pan is larger than the burner size the edges will take a little longer to heat.

I also have the Kenmore. I just did the same test alohacarol did, and my 6" pan worked on every burner. I then tried with my 3" pan (butter warmer) and got the fault message on all but the smallest burner (6").

Thank you all for your comments (and your testing)! I understand that there is a minimum pan size required to activate the burner, but I was afraid that if I put a 9" skillet on an 11" burner the pan would not get hot enough, even on high. The GE instructions state "Using a smaller pot on a larger burner will generate less power at any given setting." I want to have enough heat to saute on this burner.

On my Miele cooktop I can see no difference in power output with different pot sizes. My stove-top espresso maker is about 3 or 4" in diametre and I've never cooked espresso as fast as on this cooktop even though the diametre of the cooking hub is at least double that of the pot.