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.
Sunflower lecithin (sorry, best link I could find) is gaining in popularity as an alternative to soy lecithin because it is widely perceived to have a neutral taste and actually has superior emulsifying properties. It's a little on the expensive side, though.
Soy lecithin by itself doesn't taste horrible if you buy it as a food additive (as opposed to a nutritional supplement). It's common to find in a lot of popular frozen pasta-and-sauce dishes, such as Michelina's, which obviously must be microwaveable without the sauce curdling or separating.
Another option is Sodium Stearoyl Lactylate, if you can get hold of it. This one adds a little sweetness which is generally pleasant-tasting, or at least inoffensive. You'll find it used most often as a dough conditioner or in baked goods, but another popular use for it is in commercial sauces, although unfortunately I can't find specific examples right now.
But probably the best emulsifier for what you're trying to do here is xanthan gum. It's also used as a gelling agent, but in small quantities works as just an emulsifier. You can find it, for example, in this Bertolli Creamy Alfredo Pasta Sauce - exactly the sort of thing you're making. It's also used in the Stouffer's Alfredo sauce.
Mix in a little lecithin or xanthan gum as an emulsifier while the sauce is fresh or cooking and it's very unlikely that your sauce will separate, either in the fridge or in the microwave.
As for what you can do to specifically prevent separation while reheating, if you didn't add any emulsifiers in the first place... I really don't think there's anything you can do. If you're a slow enough eater the sauce will eventually separate right in front of you; that's just what happens when you have water and fat in the same sauce. Best to reheat on the stove top in that case, and stir frequently to prevent any further separation.
Best Answer
Microwave ovens by their nature heat unevenly. The actual microwave radiation produces wave patterns with more and less amplitude within the cooking chamber. (From a physics standpoint, these are "standing waves" produced by internal wave reflections.) This is sort of the equivalent of "hot spots" and "cold spots" in a pan.
Most modern microwaves contain some sort of rotating plate or other moving part to mitigate this unevenness by keeping the food in motion (and thus aligning different parts of the food with the "hot spots"). Over time, some parts of the food will also become very hot, and the heat from those portions will gradually distribute to the cooler portions of the food around them.
The reason I bring all of this up is that the primary advantage to cooking at a "lower power setting" in a microwave (whether it truly can change wattage or whether it merely cycles between 100% power and off, as dpolitt's answer notes) is to allow time for those hotter portions of food to distribute their heat to the cooler portions during cooking.
If you're heating something that has easy heat circulation (e.g., boiling water or other thin liquids), it doesn't much matter what setting you use, so use higher power for time efficiency.
But in many foods using a lower setting can:
In general, I'd say that there's rarely much harm in using a lower cooking setting in a microwave, other than potentially wasting some energy. As long as you lengthen the cooking time to compensate and cook the food completely, a longer time is fine. So, that's what I'd recommend if you're at all concerned about the power setting.
The only time to avoid a lower power setting is generally in foods where longer cooking times would also be avoided in other cooking methods. For example, when steaming vegetables, a longer cooking time will often result in mushiness, which may be undesirable -- whether done in a pot on the stove or in a microwave on low power. (Some cooking requires a minimum temperature to achieve normal results too, like frying. In that case, you might need to use a higher power setting. I don't tend to use my microwave for such things, so I don't have a lot of recommendations for that.)
All of that said, the primary reason to use a microwave over other cooking methods is generally speed. That's why most people tend to just use full power on microwaves for the vast majority of tasks anyway. If you're heating a food where the things listed above are unlikely to pose a major problem, using higher power may be fine.
Lastly, in many foods interrupting cooking by periodic stirring is actually more effective to distribute heat evenly than a longer microwave time at lower power. Thus, I mostly tend to use a lower power setting myself only in foods that can't be stirred or where I don't want to be bothered to take the food out and stir periodically over a longer cooking time.