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Tea Kettle v Microwave

April 18, 2011

Electricity to heat, the most epic of battlefields.  Where else would you burn something to push around electrons at 30% efficiency just to use those electrons to run something that generates heat.  Where’s a natural gas tea kettle when you need one?  Probably too dangerous for the average consumer.  Solar tea kettle?  Meh.

In any case, this battle resulted from my coworker commenting on how plugging in the tea kettle made her feel a bit guilty since it likely used a lot of juice.  So partially as a result, the tea kettle was set aside and I’d been using the microwave lately.  Of course I still happened to have the trusty Kill-a-Watt electrical usage power meter (don’t tell the Minnesota Renewable Energy Society, no one has asked for it back yet) so it was time for an experiment.

Based on my water heating experience, I know that it takes two and a half minutes in the microwave.  I learned this by nuking a mug of water for three minutes once and getting to enjoy a steam and bubble show when I dropped the tea bag into the super heated water in a smooth container.  It was kind of awesome.  As for the tea kettle, you can hear the boil develop, so timing that is easy.  Given an excuse to make tea twice in one day, it was just a matter of plugging in the meter, heating up water, enjoying chai, repeat with tea kettle.

Microwave statistics:
Approximate time to boilt – 2.5 minutes
Wattage measured while running – 1350 W
Resulting electricity usage – 0.056 kWh

Tea Kettle statistics:
Approximate time to boil – 3.25 minutes
Wattage measured while running – 860 W
Resulting kWh usage – 0.047 kWh

Tea Kettle win! TEA KETTLE WINS!

Again, similar to the previous small appliance battle, the numbers aren’t staggering.  Time is not really an issue since I can work rather than sit there watching water boil.  So the result is really a savings of 0.009 kWh per cup of water.  The Tea Kettle comes in at about 18% more efficient.  And this makes sense.  Electrical resistance heating is 95-99% efficient, meaning almost all that electricity is going to heat the water.  Microwaves on the other hand tend to range from 65-80% efficient from what I can come up with.  It wouldn’t be too hard to figure out exact efficiencies based on electricity used and the thermal capacity of water… but I’m writing this up on a Friday, so maybe another time.

Running through some other basic calculations, if one person heats up a cup of water each workday morning, the annual benefits of use the tea kettle are:

  • You use 2.3 kWh less electricity
  • You save about 25¢ (using MN approx. rates)
  • 40 lbs of CO2 aren’t released plus all other pollutants associated with power generation

Not a stand alone reason to buy a tea kettle if you don’t have one, but a good enough reason for the office to switch back to using it.

On a more qualitative side note, the microwave doesn’t induce a boil leaving plenty of air in the water.  This results in tea bags inflating like emergency rafts upon entering the water.  Since the tea kettle’s heating element causes bubbling, this doesn’t happen as much since gases have a chance to escape.  It’s just one of those interesting quirks you notice when comparing things for the first time.

Oh, on a nerdier side note, the microwave data possibly shows an example of the effects of a system’s power factor on electricity consumption.

with a power factor of one, the current is in phase

Not to dig into this too much, but simply, things that just use resistors (heaters, inefficient lighting) don’t affect alternating current very much so all energy goes into work and the power factor is 1 (or 100%).  But complicated devices with inductors and capacitors cause certain aspects of the current to get out of phase.  The result is that it takes a little extra energy to run these systems if they’re not designed properly and their power factor is less than 1 (or less than 100%).

with a power factor of less than one, the current gets out of phase

A (very) simple analogy for this is a perfect yo-yo.  In a simple resistance device or a well designed complicated one the yo-yo goes up and down on it’s own once it’s started.  But if the yo-yo is a bit out of phase, it causes you to have to move your hand and do a little work to keep it going.  Not a great comparison, but it’s the basic idea of why power factors matter for large buildings where the combination of all electronic devices is pretty complex and possibly poorly managed.

In this case, the measurements for the tea kettle were 115 volts drawing 7.5 ams which is about the 860 watts measure.  But the microwave measured 112 volts drawing 13.75 amps which is 1540 watts compared to the 1350 watts measured.  Its been a bit since the last time I did any electrical engineering, but I think this difference between 1540 watts and 1350 watts, shows that the power factor for the microwave is somewhere around 0.87 or 87%.  Interesting to me at least.

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8 Comments
  1. Aimee permalink

    You got great points there, that’s why I always love checking out your blog.

  2. Best post ever. And I’m actually scared I read the whole thing.

  3. carissa permalink

    Have you considered comparing an elective stove boiling water compared to the tea kettle? Using the electric kettle to boil the water first and then finish cooking on the stove (such as pasta or potatoes) is done in other counties a lot…especially in hostels where stove space is godly. I’ve often wondered about the energy used and would be interested in the results even though they’d be different where I’m living.

  4. TWO WORDS….power factor…we are talking guilt here, not cost. Some “power meters” show the cost to you (ignore power factor and just show watts that you will be charged for). Others show Volt-Amps (actual power consumed). The latter will not be an accurate FINANCIAL costing, but will deal with the true cost factor. However even these will show a power factor, so with a a simple calculation, you can work out what moolah it will cost. ie if your meter reads actual Volt Amps used, and also a Power Factor, then you can work out the $$ by multiplying the V-A * the PF * $/KWH. But do not do that if you are really worried about energy use. BTW, a kettle will have a PF of very nearly 1. A uWave will be lower.

  5. Mark permalink

    “Not a stand alone reason to buy a tea kettle if you don’t have one”

    Agreed. The embodied energy of producing and transporting the tea kettle + packaging — along with the the incremental impact of storing and cleaning a separate appliance — will likely dwarf the small efficiency increase of the water heating mechanism.

    In addition, very few people fill their kettles from a mug to get the water level right, and very few microwave users fill their mugs with more water than they can hold. As a result, tea kettle users introduce a behavioral inefficiency that microwave users do not.

    Given the small margin of victory in the theoretical calculations, the truth is that tea kettles are likely less efficient for all but those people who a) already own them, and b) heat only as much water as they need.

    Conclusion: Only buy a tea kettle if you have no plans to own a microwave.

  6. Ruth permalink

    Thanks for the article – it was a good read & had the answer I was looking for. When I try to explain to friends & family that tea made with a teapot tastes better than made with a microwave they never believe me until we do the taste test. Now I can explain it’s because the the tea is better infused by the water because there is a lot less O2 to buoy the tea bag, hence, more flavor. Thanks!

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