AskPablo: Microwave Efficiency

This week I will be looking into the efficiency of my microwave oven. This is part of an ongoing series in AskPablo where I am trying to determine the most efficient means for heating water. I would like to thank some of my readers for submitting excellent questions this week and I look forward publishing my answers in the coming weeks.

In order to determine the efficiency with which my microwave heats water, I will conduct an experiment. I will also try to answer the question “Is it more efficient to heat small batches consecutively or just one big batch?” First the due process: I am analyzing a Magic Chef MCD790SW, 900W microwave oven. Using my Kill-A-Watt meter (available from Europort, I have determined that the house voltage is ranging from 123.3 to 124.8 VAC today at 59.9 Hz. The ambient temperature in the kitchen is 17.4°C (63.3°F) and is expected to remain constant. I have designed this experiment to test two variables, volume and time. I have set out nine glasses in a 3 x 3 grid and have filled the top row with 100mL of water, the middle row with 150mL of water, and the bottom row with 200mL of water. After allowing the glasses and the water to reach equilibrium with the room temperature I will microwave one cup from each row at 10 seconds, 20 seconds, and 30 seconds, measuring the water temperature before and immediately after.
During the experiment I observed that, while the microwave is rated at 900W, the Kill-A-Watt meter registers between 1260W and 1314W (average: 1287W), 43% above its rating. One hypothesis that I can think of is that the 900W refers to the specifications for the microwave emitter and not the entire unit and the remaining energy is lost in transforming the 120 VAC to the proper voltage for the emitter.
With my experiment complete I now have a table of data, including beginning and ending temperature for each glass. The 100mL cups increased in temperature by 10, 22, and 35°C after 10, 20, and 30 seconds, respectively. And after 30 seconds the 100mL, 150mL, and 200mL cups were heated by 35, 25, and 20°C, respectively. These results are interesting but require further analysis in order to answer my question. I have decided to convert these values, which show a relative temperature change, to a measure of absolute energy in the system. Using the specific heat capacity value for water, 1850 J/(kg-K), I can determine, for example, that a 100mL glass at 17°C has 53.65kJ of energy (1850J/(kg-K) x 0.100kg x 290K, K=°C+273). By calculating the embodied energy in all cups before and after the experiment I can determine how much energy entered each system.
In order to compare the results I need to normalize them. This means that I need to look at the results as if all the cups had been heated for the same amount of time and had contained the same mass of water. I have decided to normalize to 200mL at 30 seconds. I do this by multiplying the 100mL results by 2.0 (200mL/100mL) and the 150mL results by 1.33 (200mL/150mL), then by multiplying the 10 second results by 3.0 (30s/10s), and the 20 second results by 1.5 (30s/20s). This gives me nine values, in Joules, all for 200mL at 30 seconds.
From these results I can conclude that it is more efficient to do multiple small batches than one big one. This result makes sense since it takes FOREVER to heat up a large plate of leftovers yet smaller plates take proportionally less time to get sizzling hot. It also appears that it is more efficient to run the microwave longer, which is probably due to the microwave emitter warming up. These results almost seem to contradict each other since smaller batches don’t take as long and may not allow the microwave to reach its prime efficiency. To reach a more definitive answer, including a point of maximum efficiency, would require a much larger experiment with many more cups than I have.
By dividing my normalized results by 30 seconds I get J/s, or Watts. This now allows me to compare the product’s specified rating with the actual amount of energy put into the water. 100mL at 30 seconds uses 432W, while my Kill-A-Watt meter measures 1287W. If these values are correct, the best efficiency recorded in this experiment is 33.6% (432W/1287W). Keep in mind that a power plant is also roughly 30% efficient, so in order to put 13kJ of energy into your cup of water in a microwave, roughly 130kJ of fuel energy is burned at a power plant (33.6% x 30% = ~10%, 13kJ / 0.10 – 130kJ).
So sit back and enjoy a nice warm cup of tea. Don’t worry too much about the energy required to make it. Instead think about that fact that most microwaves use more energy (over any given day) telling you the time than they actually do heating your food or water.
Pablo Päster, MBA
Sustainability Engineer
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