Should I default to higher or lower power when the recommended power is not an option in a microwave? I often cook takeaway dishes at home. Some of these dishes ask for 4 minutes at 850 Watt (for example, ravioli with mascarpone sauce). My microwave oven supports 750 Watt and 950 Wattt, but not 850 Watt. Should I go for 4:30 at 750 Watt or 3:30 at 950 Watt in this case (pasta with sauce)? and is this the same for all dishes, or does it depend on the dish?
One way to approach the problem is to ask the question in a slightly different way: how do I generate the same amount of energy in a 750 Watt microwave that would be produced in a 850 Watt oven over four minutes?

Watts times time will give energy, and since we are not in a Chemistry or Physics forum, let's throw units out the window.  850 Watts times 4 minutes will give us 3400 units of energy (W min to be picky).  To get the same amount of heat from a 750 Watt oven, solve the equation 750 * min = 3400 which is 4 minutes 32 seconds.  In the 950 Watt oven that would be 3 minutes 35 seconds.

Since the power button on microwaves simply cycles the oven on and off (e.g. 80% does NOT mean that a 1000 Watt oven is operating at 800 Watt, but that it is only on 80% of the time), then we can include that in the equation as well for more general problems.  So we have 

WATT * time * power = energy


This equation should allow one to determine the time or  % power to use to get the same amount of energy when cooking in an oven with a different power rating than the recipe calls for.