Which Uses Less Energy for Hot Chocolate – Microwave or Electric Kettle?

By Vernon Trollinger, November 28, 2016, Energy Efficiency

So as December’s cool winds begin throwing snow and ice our way, we find ourselves wanting to spend time with family or close friends over cups of hot chocolate. But once the small talk about weather and holiday plans has concluded, you’re invariably with the most important seasonal question of all:

Which uses less energy when making hot chocolate — a microwave or an electric kettle?

As a public service this holiday season, we want to provide THE definitive answer!

Which Uses Less Energy for Hot Chocolate - Microwave or Electric Kettle? | Bounce Energy Blog

“You know what’s more important than discussing our stock portfolios in relation to the cost of holiday shopping?! Debating the best way to make hot chocolate!”

Now, one would intuitively think that a microwave would use less energy to heat up water. After all, microwave ovens are designed to heat water. Electromagnetic radiation shoots electrons into food to induce those electrons to vibrate and thence get hot. Heating a cup of water takes just seconds.

An electric kettle, by contrast, uses electric resistance to make a metal ring get hot at the bottom of little kettle filled with water. That heat is absorbed by by the water by conduction. That’s practically stone age technology.

So why is there a controversy?

Which Uses Less Energy for Hot Chocolate - Microwave or Electric Kettle? | Bounce Energy Blog

The microwave – the symbol of modern convenience and efficiency. Or is it?!

Let’s start with some science:

  • The specific heat of water is 1 calorie/gram °Celsius = 4.186 joule/gram °C;
  • 1 gram of water = 1 milliliter of water;
  • 1 cup of water roughly equals 235 ml.

Let’s say we fill a cup with tap water that’s 10 °C (50 °F).

  • To boil that water at 100 °C (212 °F), we need to add 90 °C of heat/energy.
  • Thus, we multiply 4.186 joule/gram °C × 235 ml x 90 degrees.
  • We end up needing 88,534 joules of energy.

That 88,534 joules of energy can come from anything — flame throwers, burning sticks, or starship food replicators — but no matter what, it takes that amount of energy to raise the temperature.

How much is that energy in terms of electricity? Well, one joule per second = 1 watt. The more time it takes to heat the water, the more energy gets used. To heat up our water, we need to use a certain wattage for a certain amount of time that will be more than 88,534 joules.

What does this have to do with microwave ovens and electric kettles?

Which Uses Less Energy for Hot Chocolate - Microwave or Electric Kettle? | Bounce Energy Blog

Could someone purchase that mug for me for Christmas? Thanks.

Back in 2014, EnergyStar performed an experiment called #EnergyFaceoff . They discovered that a 1,000 watt microwave took four minutes to heat 8 ounces (236.588 ml) of water while it took a 1,500 watt kettle only 90 seconds (1:30).

So, let’s calculate the total energy used:

  • The microwave used approximately 1,000 joules/second x 240 seconds (4 minutes) = 240,000 joules.
  • The kettle used about 1,500 joules/second x 90 seconds= 135,000 joules.

Even though the kettle had a higher wattage, it used LESS total energy!

But we don’t end quite there! If we look closer at the energy used by the microwave and the kettle, we find that the microwave is much less energy efficient than the kettle. Some of the electrical energy an average microwave consumes while operating goes to other things — motors, lights, waste heat from their magnetrons, and more. Depending who you read, microwave ovens only direct about 40 to 70% of their energy to heat food or drink.

Meanwhile, an electric kettle uses a metal electric resistance ring to get hot — and it does this very quickly. Heat simply passes directly to the water inside the kettle. Thought not perfect, that’s still close to 90% efficient. With the lid closed, the heat is trapped inside the kettle, making the temperature rise faster.

And the winner is: The Kettle!

Which Uses Less Energy for Hot Chocolate - Microwave or Electric Kettle? | Bounce Energy Blog

Whew! That was enough high school chemistry for one blog post. I just wanted to drink some hot chocolate!

 

With any luck, we’ve just eliminated another controversial topic that will get you into hot water with your family and friends this holiday season.

If you’re interested in getting yourself an energy efficient electric kettle, remember that the longer it takes to heat the water, the more energy gets used. Also keep in mind the amount of water you would regularly heat since more water also takes more energy. To start, consider a powerful 1500+ watt model that’s well insulated — and then add marshmallows as needed.

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About 

A native of Wyomissing Hills, PA, Vernon Trollinger studied writing and film at the University of Iowa, later earning his MA in writing there as well. Following a decade of digging in CRM archaeology, he now writes about green energy technology, home energy efficiency, DIY projects, the natural gas industry, and the electrical grid.

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