Does microwave power influence heating time and efficiency?

We have two microwaves in the kitchen at work, and while waiting for our food to heat up, there is always debate over how much faster one of them is. I also found myself wondering whether the faster one used more or less energy to produce the same amount of heat.

In order to test this, I used a Kill-a-watt meter to measure energy use, and timed how long it took in various microwaves to heat 12 oz of water from 66.5 F to boiling (measured by the appearance of bubbles on the top surface of the water), which should have taken 0.033 kWh. I was guessing that the microwaves listed as using more power would both heat it up faster, and use more energy.

The results are below. I was surprised how inefficient the microwaves were, even if they had performed to the specifications in the manual (and they did not). Theoretical efficiency varied from 64% to 70%, but actual efficiency varied from 42% to 67%.

Essentially, higher power microwaves tend to heat up water faster,but it's not a strong linear relationship. There's almost no relationship between energy use and heating time. Also, while in general higher power microwaves use more energy to perform the same task, that wasn't as clear as expected either. For the two microwaves on the 4th floor at work (the ones I use), the faster one of the two was only 10% faster but used roughly the same amount of energy. However, I should note that since the energy meter only reported use to the nearest 0.01 kWh, the precision for energy use is pretty low. I came up with a combined metric (seconds to heat the water times energy use) to give them points both for efficiency and speed, and there was quite a bit of variety there as well.

Elapsed seconds	Total Energy Used, kWh	Actual Efficiency	Labeled Efficiency	Labeled Power Use, W	Labeled Power Output, W	Time * Total Energy
209	0.05	66.7%	65.0%	1000	650	10.450
197	0.05	66.7%	70.0%	1000	700	9.850
195	0.07	47.6%	n/a	1300	n/a	13.650
232	0.07	47.6%	64.0%	1250	800	16.240
172	0.08	41.7%	68.8%	1600	1100	13.760
217	0.07	47.6%	66.7%	1350	900	15.190
168	0.07	47.6%	64.1%	1560	1000	11.76
243	0.08	41.7%	66.7%	1350	900	19.44