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All About Microwave Ovens

A microwave oven usually uses less energy than other methods of cooking — but sometimes it doesn’t

Posted on Sep 5 2014 by Martin Holladay

A microwave oven uses less energy than a conventional oven. Even though this statement is broadly true, a microwave oven isn’t always the most efficient way to cook.

So what appliance should you use to heat up or cook your dinner: A gas stovetop? An electric-resistance stovetop? An induction stovetop? A gas oven? An electric oven? A countertop toaster oven? A crockpot? Or a microwave oven?

If all you care about is energy efficiency, it’s possible to come up with an answer to this question — but the answer will depend on the quantity and the type of food you are cooking.

If you care about the taste of your food, on the other hand, energy efficiency probably doesn’t matter. For example, if you are cooking a traditional paella, the answer is, “An open fire fueled by prunings from a vineyard” — and energy efficiency be damned.

Comparing a microwave oven to other cooking methods

A microwave oven uses quite a bit of power — between 950 and 1,720 watts — but it usually isn’t operated for long. According to a 2001 Home Energy article, “the typical residential microwave oven consumes about 110 kWh of electricity per year.” If you are paying 15¢/kWh for electricity, your microwave oven is costing you only about $16 a year to operate.

According to The Consumer Guide to Home Energy Savings, a publication of the American Council for an Energy-Efficient Economy, cooking a casserole costs 16¢ if you use a conventional electric oven; 8¢ if you use a toaster oven; 7¢ if you use a gas oven; 6¢ if you use an electric crockpot; and 3¢ if you use a microwave oven. (These calculations are based on the assumption that natural gas costs 60¢/thermUnit of heat equal to 100,000 British thermal units (Btus); commonly used for natural gas., and electricity costs 8¢/kWh. To bring these estimates roughly in line with current energy prices, the estimated costs should be increased by 40% to 100%.)

However, there are many factors that affect these types of comparisons.

Microwave ovens use less energy than conventional ovens — unless you are cooking a large quantity of food. Baking four potatoes in a conventional oven takes 2.5 as much energy as baking four potatoes in a microwave oven. But if you are cooking an entire meal for a big family — for example, a meal described by Swedish researchers as “a pot roast plus vegetables” — the conventional oven will use less energy. The food will probably taste better, too.

What if you just want to heat up a cup of water for tea? The most efficient way to heat water for a mug of tea is to put the water in a teakettle and to heat it on an induction cooktop or an electric-resistance stovetop. According to measurements made by Jennifer Mitchell-Jackson at Lawrence Berkeley National Laboratory, an electric resistance burner uses 25% less electricity than heating the water in a microwave oven. Be careful, however: if you overfill the teakettle, you may end up boiling more water than you need, negating the energy savings. While a gas stove uses more energy than a microwave oven for this task, the gas stove method will probably cost even less than the electric stove method. (Mitchell-Jackson didn't test any induction cooktops, which are even more energy-efficient than electric-resistance burners.)

A microwave oven doesn’t heat up your kitchen as much as a conventional oven, reducing the load on your air conditioner. Of course, this factor only matters during the summer.

If you can fit the food you are cooking into a countertop toaster oven, cooking it there will use less energy than a conventional electric oven.

Remember, natural gas usually costs less per BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. than electricity. The Home Energy article noted, “One hour in a gas oven costs approximately the same as 30 minutes in a microwave.”

Standby power use

The federal government currently has no energy efficiency standards for microwave ovens, either in standby mode or in operating mode.

According to a study by researchers at the Lawerence Berkeley National Laboratory, the average standby power use for microwave ovens is 3.08 watts (27 kWh per year). Measured values range from 1.4 watt to 4.9 watts.

If you don’t remember to close the door of your microwave oven after using it, the average standby power usage jumps up to 26 watts. So close the oven door!

While the standby energy use of a microwave oven isn’t huge — at 15¢/kWh, the average cost of this standby power is only about $4 per year — it’s still a fairly significant amount of power. If you only use your microwave oven for 20 minutes a week, then you are paying more for standby power than you are for heating your food.

New standards will address standby power use

The U.S. Department of Energy has recently enacted regulations that will eventually regulate the standby power consumption of microwave ovens.

The new regulation will require countertop microwave ovens to use no more than 1.0 watt in standby mode. Built-in microwave ovens will be required to use no more than 2.2 watts in standby mode. The new regulation will take effect in 2016.

Don’t worry about it

Energy used for cooking amounts to between 3% and 4% of residential energy use in the U.S. This category of energy use is dwarfed by other categories like space heating, cooling, and domestic hot water.

There is little difference in efficiency between one brand of microwave oven and another. The 2001 Home Energy article reported, “To find out just how efficient microwave ovens are, we tested ten different models using a procedure that closely resembled the International Electrotechnical Commission’s standard method for measuring microwave oven performance. … We found that the microwaves’ average efficiencies (the ratio of the electricity that went to heat food or water in the microwave divided by the total electricity used) ranged from 49% to 57%. These results were comparable to those from a 1994 study by Lawrence Berkeley National Laboratory (LBNL) which measured microwave efficiencies at 48%–60%. … Don’t worry about which brand to purchase, since the range of microwave efficiencies tends to be small.”

Here are the important points to remember about microwave oven use:

  • A microwave oven almost always uses less energy than a conventional oven.
  • You won’t discover much of a difference in energy efficiency when comparing different models of microwave ovens.
  • The amount of energy used by a microwave oven is relatively small, so any tricks you can devise to reduce the amount of energy used by your microwave won’t save much energy.
  • Food cooked by sautéing, frying, oven-roasting, broiling, or grilling over an open fire usually tastes better than food cooked in a microwave oven.
  • If you want to reduce the amount of energy you use for cooking, consider buying an induction cooktop.
  • If you live in an off-grid house, your PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. array may be producing “use it or lose it” amounts of energy during the summer. If that's the case, heating up your lunch with a microwave oven is a good way to save propane.

Martin Holladay’s previous blog: “Get Ready for Smart Appliances.”

Click here to follow Martin Holladay on Twitter.


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Image Credits:

  1. Image #1: ACP MenuMaster
  2. Image #2: American Council for an Energy-Efficient Economy
  3. Image #3: Lawrence Berkeley National Laboratory

1.
Sep 5, 2014 1:46 PM ET

give induction more credit
by Hobbit _

A cup of water for tea? Induction cooker, hands down. You don't
give them nearly enough credit for energy efficiency. They boil
water faster than the nuke with almost zero stray heat to dissipate
afterward. I would suggest that 1> people chase the linked post
about induction stovetops, and 2> someone should go remove the
spam that landed in the comments under it.

You can get a one-"burner" induction cooktop from Amazon for between
$50 and $100. The one I got, a Tatung TICT-1500TW, even came with
a cheap pot with the right sort of ferrous bottom plate to make it
work, and it happens to be the perfect shape for those "one-skillet"
type dinners or stir-frys if that's what you're into. 1500 - 1800
watts on high for most of them, but you get your oatmeal in 30 seconds.

What you don't get is that little bit of extra winter heating in
your kitchen area, but that's arguably a good thing as that's the
heat pump's job.

_H*


2.
Sep 5, 2014 2:22 PM ET

Edited Sep 5, 2014 2:24 PM ET.

Response to Hobbit
by Martin Holladay

Hobbit,
Thanks for your comments. I have edited the paragraph about Jennifer Mitchell-Jackson's mug-of-tea research to reflect your point.

Thanks also for tipping me off to the spam. It has been deleted.


3.
Sep 5, 2014 4:08 PM ET

Kettles
by Nick Welch

Stand-alone electric kettles should be more efficient than the stove top since there is much less stray heat. I just wish we had 220 wiring like Britain so we could have their fast 3000w kettles.

Since I'm wishing... how about induction ranges or cooktops that don't cost 3-4 times as much as a basic electric range. Hobbit, a single-burner standalone model is fine for some, but my kitchen doesn't really have extra space for one, and it would be obnoxious on top of our existing range (and we might accidentally burn it).


4.
Sep 7, 2014 12:37 AM ET

Echo Hobbit
by Curt Kinder

I have a standard soup lunch I consume about twice per week and it takes about 4 minutes to heat in a 1600 Watt drawer microwave, but only 100 seconds using a 1600 Watt single "burner" Burton induction cooker I also got from Amazon for under $100. My only objection to the Burton is that it concentrates all that heat in a six inch disk - continuous stirring is mandatory for the entire 100 seconds.


5.
Sep 7, 2014 11:11 AM ET

induction cooking
by stephen sheehy

I also got a cheap single burner induction cooker, primarily so I could try cooking on it before making a decision for our new house cooktop. I am very pleased with how well it works. It makes giving up gas cooking easy, and allows us to avoid the issues with gas in a tight house.
As for cost, induction is definitely more costly, but compared with a high end gas cooktop, a mid-priced induction unit isn't bad, about $1000 or so. I suppose one could calculate the payback period based on gas or electricity saved, but for me, avoiding gas is a big plus.


6.
Sep 7, 2014 4:16 PM ET

Response to Curt Kinder and Stephen Sheehy
by Martin Holladay

Curt and Stephen,
Thanks for your comments and measurements. It looks like induction cooktops win the "cup of tea" race.

But you can't bake a potato or easily reheat leftovers with an induction cooktop, so a microwave oven may still have a place in the American kitchen.


7.
Sep 7, 2014 4:54 PM ET

microwaves
by stephen sheehy

Martin-I don't think an induction cookbook will replace microwaves, but will eventually replace gas cooking. I still need the nuker for popcorn and leftovers.


8.
Sep 7, 2014 6:30 PM ET

Response to Stephen Sheehy
by Martin Holladay

Stephen,
I've always made stovetop popcorn. All you need is a heavy cast aluminum or cast iron pot with a lid.

People made popcorn for hundreds of years before microwave ovens were invented.


9.
Sep 7, 2014 7:56 PM ET

popcorn
by stephen sheehy

I've also made it on the stovetop, but since Al Gore invented microwave popcorn, that's my go to method :-)


10.
Sep 7, 2014 10:16 PM ET

I wasn't suggesting induction replaces microwave
by Curt Kinder

Each has its place. Nothing beats a microwave to quickly warm a cup of cooled coffee or random leftovers.
I just wish microwave ovens were more efficient.
When I indulge in popcorn, I use a hot air popper and dribble butter onto the stuff as it emerges.
Gas has no place in indoor cooking especially as houses get tighter, but I doubt I'd ever give up my gas grill unless I become a purist as to grilling steaks over charcoal...maybe when I retire and have that kind of time.


11.
Sep 10, 2014 5:09 PM ET

kettles and power surges
by Harry Voorhees

Electric kettles may be the highly energy efficient, but did you know that they are responsible for power surges in Britain?

http://www2.nationalgrid.com/Media/UK-Press-releases/2014/World-Cup-fans...

According to this dramatic video from the BBC, there are actually engineers standing by to power up auxiliary generating stations at the end of popular TV programs.

http://www.bbc.co.uk/britainfromabove/stories/people/teatimebritain.shtml

(If anyone was interested in the power surges caused by tea kettles, I figured it would be readers of this forum.)


12.
Sep 10, 2014 7:33 PM ET

Resistance element more efficient than a microwave?
by Carl Mezoff

The claim that using a resistance element to heat water for a cup of tea consumes fewer watts than does a microwave doing the same job is counter intuitive and almost certainly wrong. After you take your tea cup of hot water out of the microwave oven there is almost no perceptible warmth left in the oven. In contrast, after heating a kettle of water on a resistance burner you have a red hot coil of burner metal, a warm stove, and a hot steel tea pot, all of which represent significant wasted energy. How can the resistance burner method possibly match the efficiency of the microwave if the first law of thermodynamics still holds?


13.
Sep 10, 2014 9:03 PM ET

Edited Sep 10, 2014 9:12 PM ET.

@Carl
by Curt Kinder

You are right about the relatively little wasted heat energy within the cavity of the microwave oven after a short heating task.

Your analysis falls short of the mark by failing to consider the whole picture. Within the oven enclosure but not the cooking cavity lies the magnetron, which converts (some) electricity to microwave radiation. It and its associated electronics throw off enough waste heat to require a fan for forced air cooling.

A brief browse (and my own limited data) suggests "wire to water" efficiency of a microwave oven is ~50%, whereas an induction "burner" heating a low thermal mass kettle approaches 90%.

You are correct that a conventional infrared smoothtop range loses energy by heating the resistance element and ceramic smoothtop...this can be minimized by cutting power somewhat before heating task is complete; using residual heat to finish the cooking task.

The laws of physics and thermodynamics are strictly enforced, but one must consider their entire jurisdiction to correctly apply them.


14.
Sep 11, 2014 6:21 AM ET

Response to Carl Mezoff
by Martin Holladay

Carl,
You wrote, "The claim that using a resistance element to heat water for a cup of tea consumes fewer watts than does a microwave doing the same job is counterintuitive and almost certainly wrong."

As far as I know, it was not a claim; it was a measurement. For more information on the measurements reported in the Home Energy article, see Cooking with Less Gas.


15.
Sep 11, 2014 6:22 AM ET

Response to Curt Kinder
by Martin Holladay

Curt,
Thanks for your very useful explanation of the counterintuitive results that perplexed Carl Mezoff.


16.
Sep 11, 2014 6:25 AM ET

Response to Harry Voorhees (Comment #11)
by Martin Holladay

Harry,
Thanks for the links to the articles on spikes in electricity usage in Britain associated with electric kettles.

It's one more example of how the British are more civilized than Americans. When broadcast football games are over in the U.S., viewers go to the kitchen and get another beer. The British brew tea.


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