Microwave or Stove: What’s More Efficient?

Hey Mr. Green,

I have about quart of soup left over.  Is it more efficient to reheat it on my gas range or in my microwave?

—Paul, in Kalamazoo, Michigan

A microwave is a lot more efficient way to reheat than a stovetop, partly because a stovetop wastes a fair amount of heat. According to the EPA, the microwave uses 80 percent less energy when heating up relatively small amounts. Although the microwave needs a lot of electricity, it’s a relatively short burst of power. As the immortal Yogi Berra might put it, “A watched pot never boils, but it boils quicker in a microwave.”

Of course you can make regular gas or electric cooking somewhat more efficient by matching the size of the burner to the size of the pot, and, well, duh, keeping a lid on unless the recipe forbids. The EPA, for example, warns that using a six-inch pot on a eight-inch burner can waste a goodly portion of a burner’s heat.

Although cooking accounts for only 3 percent of the total greenhouse gas emissions from residences, every bit we can cut is significant. Shaving off a few percent of each of the nine major residential-use categories can result in substantial savings on your energy bills. In Michigan, at your present utility rates, to heat your soup in the microwave would cost you about 1.9 cents, while the gas burner would run you 6.8 cents.

For the mathematically inclined, here’s the story on microwave v. stovetop. Assuming you store your batch of soup in your fridge—a practice I strongly recommend—you’ll be boosting the temperature from the typical fridge reading of about 35 to 38 degrees Fahrenheit to the boiling point, or about 180 degrees.

According to USDA’s kitchen mavens, you can heat a cup of ice-cold water to boiling in an 800-watt microwave in 2.5 minutes. (In the spirit of scientific inquiry, I independently verified this by testing the time-to-boil in my own microwave.) So you get a cup from freezing to the boiling in 2.5 minutes. To find out how much energy that took, you calculate the amount of electricity used in kilowatt hours (kWh) by taking the device’s wattage and multiplying it by the time. So 2.5 minutes = 0.042 hours, and 800 watts is .8 kW. Multiplying kWh capacity by time we get .8(0.042) = .034 kWh to heat the cup of water.

Now to gas, for which we’ll start with the British thermal unit (BTU). (You could do this all in Joules and calories if you’re metrically inclined.) A Btu raises the temperature of a pound of water by one degree Fahrenheit. To raise a cup, or .5 pounds of water from near freezing to 212 degrees we’d need .5 (180) Btus = or 90 Btus. However, since range top burners are only about 50 percent efficient, you’ll need 180 Btus total. Since kWh contains 3412 Btu, to heat a cup the stove would use the equivalent of 180/3412= 0. 0.35 kWh. This makes it seem that the microwave at .034 kWh is 10 times more efficient. But remember, when electricity is generated by fossil fuels, typically about two-thirds of the original energy is lost, so the total energy needed to run the microwave for the stated time would be .1 kWh, or about one-third as much total energy, or about 70 percent less, close to the EPA estimate cited above.

Got a question? Ask Mr. Green!

 

--illustration by Little Friends of Printmaking