Heating two servings of something takes less than twice the amount of time required for heating one.
Different foods absorb microwaves to different degrees. Water and fats absorb microwaves efficiently, while proteins and carbohydrates don’t absorb much at all. That’s why different foods require different amounts of time to heat or cook. Furthermore, the microwave generator (the magnetron) varies its output according to how big a “load” of absorbing material (a.k.a. food) is in the oven.
Here’s a very rough way of looking at the problem. Let’s say that your particular food absorbs, and turns into heat, a certain percentage of the magnetron’s microwave output. But when there are two servings in the oven, neither one is being exposed to the magnetron’s full output of microwaves; each gets only the unabsorbed “leftovers” from the other. So naturally it will take more time to heat two of them than to heat one. But how much more?
I’ll spare you the arithmetic, but the way it works out is that if one of your servings absorbs, say, 40 percent of the microwaves that it is exposed to, then it will take only 25 percent more time to heat two portions than to heat one. This time increase won’t always be 25 percent; it will be different for different foods that have different appetites for absorbing microwaves.
I tested these ideas with my own “smart” microwave oven, which has pre-programmed cycles for various common heating and cooking chores. For “Heating a Beverage,” for example, the oven first asks me to press a button to tell it how much liquid I want to heat. It then begins its pre-programmed heating cycle for that amount. I timed the heating periods, and here’s how long they lasted: for 0.5 cup, 30 seconds; for 1.0 cup, 50 seconds; for 1.5 cups, 70 seconds; for cups, 90 seconds. You can see that the first half-cup requires 30 seconds but that each additional half-cup requires only 2 0 additional seconds. Two cups took only 1.8 times as much time (90 ÷ 50) as a single cup.
Another example: For “Baked Potatoes” (they’re actually not being baked, but I’ll let that go), the oven cooks one potato in 4 1/2 minutes, and adds 3 minutes and 10 seconds for each additional potato. Putting it another way, two potatoes take only 1. 7 times as long as a single potato; three potatoes take times as long, and four potatoes take 3.1 times as long.
Lacking an omniscient oven that has pre-programmed cycles for every conceivable type and amount of food, all we can do is make an educated guess. For two servings, your first guess should be about one and three-quarters times the time required for a single serving. Doubling the time would be likely to overheat your food, perhaps making it splatter or dry out. It’s best to be conservative, because you can always zap it a little longer.
Microwave ovens are very complicated devices, truly understood only by their electrical-engineer designers. The analysis above, based on a constant percentage of microwave energy being absorbed by each portion, is oversimplified. But I didn’t think you wanted to get involved in load impedances, cavity resonances, and loss constants. And neither did I, for the plain reason that I don’t understand them.