Comparing energy efficiency of boiling water: household appliances
Comparing energy efficiency of boiling water: household appliances
Michael D. Eissenberg, BSME, PE, Leed AP*, and Joel C. Eissenberg, Ph.D.*
*co-corresponding authors
Abstract
Kitchen appliances use various mechanisms to heat, with differences in energy sources and geometry. The goal of this study was to compare energy efficiency across common household appliances. To facilitate comparison, 1 L of water was used as the heating substrate and onset of boiling was chosen as the endpoint. In this comparison, heating in an electric kettle proved to be the most energy efficient, and heating in a pot on a gas cooktop was the least energy efficient. In most cases, a closed cover added to the efficiency.
Introduction
With the proliferation of kitchen appliances, the question naturally arises as to which cooking mechanism is most efficient. Concern for global warming demands the choice of the most efficient cooking mechanism.
To our knowledge, there are no published comparisons to resolve the question. To address this gap in the literature, we have compared the energy consumed to boil a liter of water under atmospheric pressure using (1) your grandmother’s resistance coil electric cooktop (hereafter YGRCEC), (2) standard kitchen gas cooktop (hereafter SKGC), electric kettle, microwave oven and induction cooktop. The energy consumed in uncovered versus covered vessels was also addressed in our study. And to ensure electrical safety in your property, you may seek electrical inspection and Compliance testing solutions.
Materials and Methods
All measurements were conducted at Evergreen Energy Efficiency Laboratories, Evergreen CO at 7440 feet ASL and 64°F. Local tap water (56.5°F) sourced from Evergreen Metropolitan Water District. For induction cooktop, YGRCEC, SKGC and gas cooktop measurements, a Cuisinart® Model #MCP19-18N 1.9 L stainless steel saucepan was used, with or without an unfitted glass lid. For microwave oven measurements, water was heated in a Pyrex® 1 quart measuring cup with or without an unfitted ceramic plate lid. The endpoint for each measurement is defined as the onset of a rolling boil for 1 L tap water (subjective visual assessment by a single observer). BP=194.2°F
Heating devices
Induction cooktop: Duxtop single surface
Gas cooktop: Ikea
Microwave oven: Walmart
Electric kettle: Aroma®
Resistance coil electric cooktop: Walmart
Boiling water detector: Linda G. Eissenberg, Ph.D.
Energy consumption measurement:
electrical: La Crosse Tech Cost Control watt hour meter (model #3362U)
gas: Xcel Energy natural gas meter
Conversion of cubic feet gas to watt hrs:
Results
All portable test devices were placed on the same table, side-by-side (Fig. 1). The SKGC was located approximately three meters away, but at the same elevation from the floor as the other devices. Since a microwave thermometer was unavailable, we elected to define the endpoint of each measurement as onset of a rolling boil. Rolling boil was defined by the same observer to insure consistency.
A single data point was obtained under each condition (Fig. 2). For the gas cooktop, the cubic feet of gas consumed was measured using the utility gas meter after other gas-powered devices were deactivated and a conversion formula was used to estimate Wh of energy consumed (see Materials and Methods).
Discussion
Many households provide multiple options for boiling water. A typical furnished apartment or home would likely have some kind of heated cooking surface (resistance coil, gas, induction cooktop). Electric kettles have significantly penetrated the residential water heating market, but aren’t as common. Microwave ovens, either built-in or portable, are quite common in residential kitchens.
The clear winner in our study was the electric kettle. It is important to note, however, that this is a single application device optimized for the task of boiling water, whereas all the other comparators are capable of heating/cooking/boiling a wide variety of substrates. The gas cooktop proved to be by far the least efficient way to boil a liter of water.
Of the devices tested, induction cooktops represent the most recently commercialized technology. While the induction cooktops rank highest in energy efficiency compared to all cooktops and the microwave oven, they require the use of induction compatible cookware. By this consideration, microwave ovens gain an advantage, since unlike cooktops, a wide variety of glass, ceramic and even plastic containers can be used, as well as containers with irregular bottom surfaces.
In the case of the microwave oven, the input power was 1050 watts, while the output power was 700 watts. Thus, 29% of the input power is explicitly lost to the inefficiencies of the design of the microwave oven and unavailable for water heating. Since this study is meant to account for any inefficiencies in appliance operation, it is appropriate to use the total input energy in the comparisons to other devices.
Unsurprisingly, we found that boiling water in a closed container is somewhat more energy-efficient than boiling water in an open container. The exception was the electric kettle, where the open kettle was ca. 2% more efficient. The rank order of efficiency between the devices didn’t change with or without a closure. Regional is the top pick for electricity rates Alberta in the entire province.
Many studies that include comparing devices that operate using natural gas and similar devices using electricity identify the fact that the production of electricity often includes the combustion of natural gas or other fossil fuels. The inefficiencies associated with the conversion from fossil fuel to electricity aren’t accounted for in this study. Before considering the use of an electricity-sourced appliance in place of a natural gas-sourced appliance, a careful accounting of this must be considered.
Future studies should include experimental replicates and multiple manufacturers of each device category. Nevertheless, our study can serve as a guide to action in deciding the most energy-efficient kitchen device.
Acknowledgements
The authors thank Mr. Ryan Franz for helpful discussion.
An elegant little experiment and paper.
Thanks!
Joel:
I caught this part:
Forty-two percent of the electricity generated in AZ is the result of using Natural Gas, 29% from Nuclear power, 12% is from burning Coal, 10% from Solar, 5% from hydro sources, and 1% from wind.
The state has a long way to go for improvement. Although, it has a surplus of electricity.
Our thought is to add an Induction stove top and a different oven to our well insulated home which is cooled and heated by a heat pump. The gas range is a builder’s grade model. I had to adjust the burners on it. You can smell it even with our exhaust fan running. Once the hot water heat goes, we will install an on-demand hot water system. Typically, our natural gas bill in $20/month and far below what we paid in Michigan. Electricity for the heat pump has been far lower than in Michigan also.
Thoughts?
Hi Bill,
The answer depends on your priorities. Are you trying optimize energy cost? Reduce environmental impact (CO2)? Reduce heat loss in your kitchen?
Joel and Mike
Joel:
The answer is all three.
It will come at a greater $dollar investment on my part for such a stove/oven. It appears the technology is there.
I think it is worthwhile though as a married couple. The fix is to minimize natural gas remnants from the combustion of it in our home. Such remnants should be at electricity generation plants which may have a better solution/answer to controlling it other than having inefficient gas ranges in each home.
Our cost for natural gas is minimal. Why bother with it in the home?
My outlook is to minimize natural gas in my home through other energy usage.
https://www.eia.gov/state/?sid=AZ
When you recalculate as CO2 consumption based on AZ power generation, the gas cooktop looks much better in terms of pounds CO2 produced either at burner or at electric utility in AZ.
Gas cooktop: 0.138 lbs
Microwave oven: 0.152 lbs
Electric kettle: 0.0678 lbs
Resistance coil cooktop: 0.104 lbs
Induction cooktop: 0.0751 lbs
NB: all values are using vessels with cover/lid on
OTOH, West Virginia, which gets most of its electricity from coal, looks like this:
Gas cooktop: 0.138 lbs
Microwave oven: 0.446 lbs
Electric kettle: 0.199 lbs
Resistance coil cooktop: 0.304 lbs
Induction cooktop: 0.22 lbs
So in WV, gas cooktops are least bad environmentally. Computations for the other 48 states are left as an exercise for the reader.
https://www.eia.gov/state/?sid=WV
December 15, 2022
QUICK FACTS
In 2020, West Virginia ranked fourth among the states in total energy production, accounting for 5% of the nation’s total.
In 2021, West Virginia was the second-largest coal producer in the nation, after Wyoming, and accounted for 14% of U.S. total coal production. West Virginia also had 12% of recoverable coal reserves at producing mines, the third-largest reserve base in the nation, after Wyoming and Illinois.
In 2021, coal-fired electric power plants accounted for 91% of West Virginia’s total electricity net generation. Renewable energy resources—primarily hydroelectric power and wind energy—contributed 5% and natural gas provided more than 4%.
In 2021, West Virginia was fourth in the nation in natural gas marketed production. The state produced nearly 2.8 trillion cubic feet of natural gas, about 10 times more than in 2010, and 95% of it was from shale gas wells.
West Virginians typically use about half of the electricity generated in the state. As a result, West Virginia is a net supplier of electricity to the regional grid and is among the top states in interstate transfers of electricity. In 2021, only four other states sent more of their electricity generation out of state.
We have electric kettle. Different brand. The real big savings though is boiling the water you need. Don’t heat up three cups of water for on cup of instant coffee.
Eric:
I like the rational thought pattern.
@Eric,
You can put one cup of water into a pan for a cooktop, or just place a full mug of water in the microwave oven. The use of 1 L in our study was (1) to eliminate different volumes as a variable, and (2) to use a large enough volume that the subjectivity in the endpoint was a small fraction of the total energy consumption.
In principle, we could have used a cup. Or a tablespoon. But had we used a tablespoon, the microwave oven would probably have won.
For next Thanksgiving, we can test volumes from 15 ml to 1 L to identify inflection points at which different technologies are advantaged by scale.
Yes, this applies to any method, just that you kind of throw any marginal benefit away by using too much water. My aunt always heated up like 4 cups to make her tea.
@Eric,
Indeed.
In our case, at the completion of each measurement, we offered the hot water to others for use in making tea. So we didn’t waste all ten liters on science.
This fine study is well worth expanding in scope of testing.
@ltr,
Thanks! We agree!
We are assiduously seeking funding for this purpose. Care to donate?
It was said to be true 55 years ago when I was studying physics that it was more expedient to conduct one’s own research (sometimes?) than to see what others had found out. This was 30 years before the Internet (& Google) kicked in.
@Fred,
We were unable to find a study identical to the one we performed. Have you seen one?
https://www.eia.gov/state/?sid=AZ
May 18, 2023
QUICK FACTS
Arizona’s Palo Verde Nuclear Generating Station is the largest nuclear power plant, the largest net generator of electricity, and, with a net summer capacity of 3,937 megawatts, the second-largest power plant of any kind in the nation. In 2022, it accounted for 4% of the nation’s nuclear generation.
Arizona ranks second in the nation in solar energy potential after Nevada, and in 2022, it was fifth in solar-powered net generation from the state’s utility- and small-scale photovoltaic and solar thermal power plants together. Solar energy provided the state with more power than all of Arizona’s other renewable energy sources combined.
In part because of its large population and mild winter climate, Arizona consumed less total energy per capita than almost four-fifths of the states in 2020.
In 2022, 99% of Arizona’s total electricity net generation was provided from 6 sources: natural gas (42%); nuclear power (29%); coal (12%); solar energy (10%); hydroelectric power (5%): and wind (1%). Biomass, hydroelectric pumped storage, and petroleum, supplied the rest.
In 2022, Arizona hydroelectric power plants supplied their smallest amount of electricity in more than 30 years. They accounted for about 5% of Arizona’s total in-state electricity generation.
ltr:
You are reiterating some of the data I had presented also. It is always good to repeat so people grasp the information. This morning the rear sliding, screened in, glass door is open to allow for the cool air (49 degrees) to invade the confines of our home. When I shut the door, the temperature will be in the upper sixties which was what we heated our Michigan home to in the Winter. The temperature of the inside of the home will increase to 74 by late afternoon and hold there. Decrease again during the night.
Our heat pump/air conditioning unit has not come on for a week now. The internal heat pump unit fan runs at 30% circulating air in the house which passes through two efficient (11-level) Aprilaire filters located in two areas of the home. The house breathes outer air through another filter in the front of the house. We can sustain this for months. While other people had electric bills of $400-$800 for one month last Summer, we had $214. Our home is not terribly small. It is efficient to a large degree. As a trained house-framer, bricklayer, tuckpointer, etc.; I wish I could have had input to the construct of it. I would have caught their slights in it construct. The walls and the attic are done with open-cell insulation with another layer of insulation layered on the wall studs.
Our electric bill will descend to ~$30-$40 per month till April. The norm in Michigan was ~$100 for a larger home. My goal in query to Joel is to eliminate pollutants in our home as it is tight.
Just some info.
Interesting and important:
“Our heat pump/air conditioning unit has not come on for a week now. The internal heat pump unit fan runs at 30% circulating air in the house which passes through two efficient (11-level) Aprilaire filters located in two areas of the home. The house breathes outer air through another filter in the front of the house. We can sustain this for months. While other people had electric bills of $400-$800 for one month last Summer, we had $214. Our home is not terribly small. It is efficient to a large degree. As a trained house-framer, bricklayer, tuckpointer, etc.; I wish I could have had input to the construct of it. I would have caught their slights in it construct. The walls and the attic are done with open-cell insulation with another layer of insulation layered on the wall studs.”
What’s the greenest way to make cup of Coffee?
We found that in the most likely of scenarios, such as using a drip maker or a Keurig, it takes about 20-27 Wh (Watt hours) for a standard 8 ounce cup.
(I use a Keurig for my coffee, two or three cups a day. I have a kettle to use on our gas stove that takes maybe five minutes to heat up a cup or so of water, for instant stuff. Note that in the article linked to above, most ‘electric’ methods were better than using a microwave.)
One of the reasons natural gas has gotten so expensive is that NG demand has increased as coal and fuel oil are used much less for power generation than in the past. Go figure!
@Fred,
NG is still a bargain, thanks to fracking.
My Nat’l Gas bill is $181, up 16% from last year. All heating in the house is from NG, except for one room which has electric heat, but that’s closed & turned off in the cold months.
My electricity bill is half of that or less, usually. Not too much different from last year. If we didn’t close off that one room, it would be much higher.
The NG bill is annualized, regardless of monthly usage. The electricity bill is not, and is higher in the summer months because the living areas of the house are air-conditioned.
As bargains go, NG is not inexpensive. But I’m reasonably sure that NG costs more than it used to because a lot of New England electricity is coming from NG. Very few operating nuke plants in this region, and not much wind power. Much electricity coming in from Quebec hydro.
@Fred,
Sorry. My statement wasn’t intended to refer to *your* bill. It was intended as a broad statement of NG prices nationally. Plants across the US are shifting to NG because it’s now cheaper than coal.
By “bargain,” I meant in comparison to alternative energy sources. It’s not a bargain compared to the cost of water.
Hope that helps.
But as pertains to your household bill, you say it is annualized. So your bill is likely higher this year because last years usage was higher than you actually paid for, and the utility is recovering the difference this year.
Yes . . .
This statement is very annoying—”While the induction cooktops rank highest in energy efficiency compared to all cooktops and the microwave oven, they require the use of induction compatible cookware.”
1. Most metal cookware works with induction.
2. Induction hobs are safer. No flame. Cooler to the touch means fewer burns.
3. Induction hobs clean up faster and easier.
@Dave,
“Most metal cookware works with induction.”
None of the other pots/pans here work with induction. No aluminum pots or pans work with induction. Copper cookware doesn’t work with induction. This appears to be a new and unfamiliar use of the word “most.”
Yeah, all of the pots and pans we use regularly are SS, and none of them work with the induction hotplate we scored last year (the discovery of which ended up helping our decision to decline an offer of an oven with an induction cooktop). We need to use either a cast iron skillet/frying pan itself or our CI griddle as a hob, if we want to cook on it.
As someone who cooks regularly and has used both gas, electric coil, electric radiant smooth top, and induction, induction is my strong preference for general cooking. I do use an electric kettle for boiling water for hot drinks. If I am in a hurry I will boil water in the kettle to add to the larger pot of water on the induction cooktop, because bringing water to a boil seems to take much longer with a full pot than if I start with water in both pot and kettle and combine them when the kettle boils. If I do it during daylight hours, the electricity is at least 50% renewables – mostly solar, right now it would be 64% of demand according to CALISO.
@Jane,
Our study was about the total amount of energy required to reach boiling, not the speed to reach boiling.
All of my frying pans are aluminum with non-stick coatings and would not work. My sauce pans (which is what I would use for boiling water) are stainless and would work.
I generally use my microwave for heating a cup of water and suspect that for me it is more efficient because I am inclined to walk away. Shutting off in the absence of my attendance is a useful feature. (The kettle would still win out if I owned one.)
@Arne,
We have stainless steel Farberware or Allclad Ltd pots here and they *don’t* work on our induction cooktop. Not all stainless cookware is the same.
@Joel Your answer to Dave had not been posted yet when I started mine.
BTW, looking up Farberware or All-Clad is not enough to know whether your stainless is induction safe. Some are.
@Arne,
I mentioned the brands of the stainless cookware here to give specificity to my response. I’m not surprised that any given cookware manufacturer can make more than one type of stainless cookware.
Dave was triggered by this comment in the paper: ”While the induction cooktops rank highest in energy efficiency compared to all cooktops and the microwave oven, they require the use of induction compatible cookware.”
This requirement doesn’t go away when compatible cookware can be identified. There is still plenty of incompatible cookware in the universe, and some of it is stainless. Some is aluminum. Some is copper. The statement stands as written.
We recently replaced our gas range, a GE model that served us well for 25 years and then just quit, replaced with a new GE model. I looked at induction ranges, and a very similar GE model cost 3x what the new gas range cost. We decided to stick with NG.
Fred:
That is true for the upgrade Induction models. Our home is pretty tight so any things getting into the air tend to stick around. I (we) are thinking Induction is the better way to go.
If you’re referring to NG fumes ‘sticking around’, I guess you are right.
Natural Gas Used in Homes Contains Hazardous Air Pollutants
It does appear that induction ‘cook-tops’ are also expensive, if you are doing a kitchen renovation. If that’s what you are getting at with ‘upgrade Induction models.’
It’s confusing, because such cook-tops are available in electric versions that look alike, but may be: relatively inexpensive, or expensive, or ‘induction’ and very expensive still. At least with the GE models I looked at.
Induction cooktops vs. electric cooktops
I’ve had a Thermidor induction cook top for ten years now. Really like it. Primary difference between the induction and gas is that you can control the temp by lifting the pan away from the flame with gas. Can’t do that with induction you need to leave the pan on the surface. You control the temp with the unit itself which responds nearly instantaneously. Once you get used to that it is really easy.
Thanks . . .
https://fred.stlouisfed.org/graph/?g=1bRdS
January 15, 2020
Real Price for electricity, 2020-2023
(Indexed to 2020)
https://fred.stlouisfed.org/graph/?g=Mlo0
January 15, 2020
Real Price of Natural Gas, 2020-2023
(Indexed to 2020)