Tuesday, October 14, 2014

Microwave vs Electric Kettle: which uses more energy for heating water?

I appreciate a good cup of tea in the morning.  My saintly father-in-law always has instant oatmeal for breakfast.  And the ineffable Mari likes her occasional hot chocolate.  All of these require piping hot water, and since we're all about saving energy, we wanted to know the most energy-efficient way to heat up a few cups of hot water.  So we staged a face off between two common kitchen appliances: a small microwave oven and an electric kettle.

The Contestants

Our microwave is a Sharp model R-308JS with a capacity of 1.0 cubic foot, rated at 1100 Watts.
Our electric kettle is an Aroma 1.5-liter Electric Tea Kettle, model AWK115S.

Measuring Energy

For both the microwave and the kettle, we want to measure how much energy is required to bring one pint of water (that's sixteen ounces or 0.47 liters) to a boil. In both tests, the water starts at room temperature of 72°F (22°C).

We can use the BlueDot application as a simple watt meter to measure the power draw of each appliance. The Current Energy Use screen has a nifty feature to measure the power consumed by an appliance: tap the screen to "zero out" the standby power consumed by your home, then turn the appliance on.  In a few moments, the screen displays the difference between the standby power and the total power currently being consumed. This number reflects how much power the appliance is consuming.

Here's how you use the BlueDot app as a power measuring tool:

First, click on "Current Energy Usage" to bring up the big meter.  Here's what it will look like.  For our experiment, we've selected "Watts" rather than "$$" since we want to measure watts.  We can see that our house is consuming 339 watts right now.
Tap on the center of the meter in order to se the "zero point".  It will draw a turquoise pointer that remembers the power at the moment you tapped it.

Now turn on the appliance.  In this case, we've turned on the water kettle.
After about ten seconds, you will see something like this.  The number displayed in turquoise is the difference between the immediate power draw in your home and the zero point set up when you tapped on the screen.

Here, you can see that the water kettle is currently consuming 1436 watts.
Here, we've turned off the kettle.  It's significant that the pointer has returned to within 1 watt of the zero point: this suggests that no other major appliances turned on or off while we were doing our tests, which would invalidate our measurements.

So in this case, we're confident that the kettle was consuming 1436 watts while it was running.

Power versus Energy

The astute reader will notice that the Current Energy Usage screen tells us how much power an appliance is drawing, not how much energy is being consumed. The distinction is sometimes a source of confusion, but it's not complicated.

Power is usually measured in watts or kilowatts (or if you're a real nerd, joules per second). You can think of power as the force of water coming out of a hose: a dribble has almost no power, the stream from a fire hose has a lot of power.

Energy is usually measured in watt-hours or kilowatt-hours (and for the nerds among us, watt-seconds or joules). Using the water analogy again, you can think of energy as a quantity of water: a thimble-full is a small amount of energy, a swimming pool is a lot.

In summary:

  • Power: "how hard the water is squirting" measured in watts
  • Energy: "the quantity of water that you've used" measured in watt-hours

The Tests And Results

For the microwave, we put a glass containing one pint of water at room temperature and measured the power consumed while the microwave was on and how many seconds it took for the microwave to bring the water to boiling. We repeated the process for the electric kettle. (No, we did not put the glass inside the kettle, just the water...)

By multiplying watts times seconds, we get the energy measured in watt-seconds. And by dividing watt-seconds by 3600 (the number of seconds in an hour), we get watt-hours, which is what we're after.

The results are tabulated below.


This shows us that the electric kettle consumes about 32% less energy than the microwave.

But before you run out and purchase an electric kettle to augment your appliance collection, consider this: at $0.11 per kilowatt-hour (the national average price of electricity), heating those 16 ounces of water in the microwave will cost you about a penny. Heating the same amount of water in an electric kettle will cost you about 0.7 cents. You can purchase an electric kettle like the one shown above for about $30, but it will take nearly ten thousand cups of morning tea (see Note) before it pays for itself.

Nonetheless, I really like my electric kettle.

Note: $30 / ((88.5 - 60.2) * $0.11 / 1000) = 9637 mornings, or over 26 years.

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