Prepackaged microwave food typically says to microwave on high for n seconds. It usually says that the instructions are meant for an 1100 watt microwave (give or take).
My microwave is only 950 watts.
What is the equation for me to get the new time? Is it time x (1100 / 950)?
Best Answer
Simple answer
The same equation, because the power or wattage between both your microwave and the reference/recipe microwave are close enough that your formula would be a decent estimate of time with some "tolerable" error (and will work for any case this happens).
Technically speaking this means you just estimated cooking time by thinking it would behave linearly when the power between microwaves isn't that different, which is a decent estimate with some error (but also not really true and mistaken for whenever this doesn't happen).
Elaborate answer
For your specific case (owning a 950W microwave and having 1100W instructions) even though mathematically speaking an 86% difference (950/1100*100) in wattage seems like much, when you take into any consideration the available data like this one (source)
Notice that you can literally validate your formula by checking the data. So if for example you have a 1000W and you have a recipe with 3.10 minutes of cooking time tested with 1200W microwave, your formula estimates the cooking time to be 3.10*1.2=3.72 minutes, but the actual time in the table implies you're off because the real time is 4.03 minutes. So like I said before, decent estimate with tolerable error.
However, in reality cooking time in a microwave behaves non-linearly and thus, whenever the difference of wattage gets larger your formula doesn't work.
To give a formula for that case is harder when you realize the amount of variables to take into account, so this is why tables like the one above exist to give you some perspective of the cooking times. One would think that the table above solves the whole problem, but realize that those are cooking times for a specific food (baked potato); so if you have a food with no data I'd say experimentation is advisable at your own risk :D.
PS: I'm a mathematical engineer who likes cooking and just wanted to provide another perspective on the matter. Hope it helps!