Precise microwave oven control
Only a few microwave ovens control the microwave power level on essentially a continuous basis.  That is, if you set the power level at 30%, the oven runs at 30% power output.  Most are based on turning the microwave power on at 100% for a given portion of a fixed time increment, like 30 seconds or 1 minute.  Then the microwave power shuts off for the remainder of the time increment, and the cycle repeats.  

My microwave oven is based on a 30 second cycle, so the power level setting translates to 10% increments of 30 seconds.  At 50% power level, there is microwave power at 100% for 15 seconds, none for the next 15 seconds, then the cycle repeats.  If the time is set to less than the resulting power-on time, the oven stops when the time runs out.

So in my first example, with a 50% power level, the material gets 15 seconds of full power whether the timer is set for 15 seconds or 30 seconds.  In the second example, with a 20% power level, the material gets 6 seconds of full power at the start of the first 30 seconds, then another 5 seconds of full power before it shuts off at 35 seconds (almost double the energy despite the small time increase).

To get precise amounts of microwave power using the power level setting, you need to base it on the cycle time of your oven.  You can tell what the cycle is for your oven by listening to it cycle the power on and off and check the oven's clock to see the time between power-on cycles.  If you need brief bursts of microwave power, you may need to use a very low power level (and the process may take a long time).

The way the power level actually works makes it not particularly useful for short-duration requirements.  For those, I typically do what Chris H suggests in the comment -- do it manually at full power to get exactly the duration needed.  The main benefit of the power level setting is when you need a lot of cycles of short duration, and doing it manually would be tedious.  Understanding the cycle action lets you ensure that the duration at full power doesn't exceed a certain number of seconds at a time.