Induction Cooking: How It Works

Here's the Basic Idea

"Cooking" is the application of heat to food. Indoor cooking is almost entirely done either in an oven or on a cooktop of some sort, though occasionally a grill or griddle is used.

Cooktops--which may be part of a range/oven combination or independent built-in units (and which are known outside the U.S.A. as "hobs")--are commonly considered to be broadly divided into gas and electric types, but that is an unfortunate oversimplification.

In reality, there are several very different methods of "electric" heating, which have little in common save that their energy input is electricity. Such methods include, among others, coil elements (the most common and familiar kind of "electric" cooker), halogen heaters, and induction. Further complicating the issue is the sad habit of referring to several very different kinds of electric cookers collectively as "smoothtops," even though there can be wildly different heat sources under those smooth, glassy tops.

As we said, cooking is the application of heat to food. Food being prepared in the home is very rarely if ever cooked on a rangetop except in or on a cooking vessel of some sort--pot, pan, whatever. Thus, the job of the cooker is not to heat the food but to heat the cooking vessel--which in turn heats and cooks the food. That not only allows the convenient holding of the food--which may be a liquid--it also allows, when we want it, a more gradual or more uniform application of heat to the food by proper design of the cooking vessel.

Cooking has therefore always consisted in generating substantial heat in a way and place that makes it easy to transfer most of that heat to a conveniently placed cooking vessel. Starting from the open fire, mankind has evolved many ways to generate such heat. The two basic methods in modern times have been the chemical and the electrical: one either burns some combustible substance--such as wood, coal, or gas--or one runs an electrical current through a resistance element (that, for instance, is how toasters work), whether in a "coil" or, more recently, inside a halogen-filled bulb.

Induction is a third method, completely different from all other cooking technologies-- it does not involve generating heat which is then transferred to the cooking vessel, it makes the cooking vessel itself the original generator of the cooking heat.

(Microwaving, an oven-only technology, is a fourth method, wherein the heat is generated directly in the food itself.)

How does an induction cooker do that?

Put simply, an induction-cooker element (what on a gas stove would be called a "burner") is a powerful, high-frequency electromagnet, with the electromagnetism generated by sophisticated electronics in the "element" under the unit's ceramic surface. When a good-sized piece of magnetic material--such as, for example, a cast-iron skillet--is placed in the magnetic field that the element is generating, the field transfers ("induces") energy into that metal. That transferred energy causes the metal--the cooking vessel--to become hot. By controlling the strength of the electromagnetic field, we can control the amount of heat being generated in the cooking vessel--and we can change that amount instantaneously.

(To be technical, the field generates a loop current--a flow of electricity--within the metal of which the pot or pan is made, and that current flow through the resistance of the metal generates heat, just as current flowing through the resistance element of a conventional electric range's coil generates heat; the difference is that here, the heat is generated directly in the pot or pan itself, not in any part of the cooker.)

How Induction Cooking Works:

1. The element's electronics power a coil (the red lines) that produces a high-frequency electromagnetic field (represented by the orange lines).

2. That field penetrates the metal of the ferrous (magnetic-material) cooking vessel and sets up a circulating electric current, which generates heat. (But see the note below.)

3. The heat generated in the cooking vessel is transferred to the vessel's contents.

4. Nothing outside the vessel is affected by the field--as soon as the vessel is removed from the element, or the element turned off, heat generation stops.

(Image courtesy of Induction Cooking World)

(Note: the process described at #2 above is called an "eddy current"; heat is also generated by another process called "hysteresis", which is the resistance of the ferrous material to rapid changes in magnetization. The relative contributions of the two effects is highly technical, with some sources emphasizing one and some the other--but the general idea is unaffected: the heat is generated in the cookware.)

(You can see what such a coil and its associated electronics looks like in the image at the right.)

There is thus one point about induction: with current technology, induction cookers require that all your countertop cooking vessels be of a "ferrous" metal (one, such as iron, that will readily sustain a magnetic field). Materials like aluminum, copper, and pyrex are not usable on an induction cooker. But all that means is that you need iron or steel pots and pans. And that is no drawback in absolute terms, for it includes the best kinds of cookware in the world--every top line is full of cookware of all sizes and shapes suitable for use on induction cookers (and virtually all of the lines will boast of it, because induction is so popular with discerning cooks). Nor do you have to go to top-of-the-line names like All-Clad or Le Creuset, for many very reasonably priced cookware lines are also perfectly suited for induction cooking. But if you are considering induction and have a lot invested, literally or emotionally, in non-ferrous cookware, you do need to know the facts. (Check out our page on Induction Cookware.)

(And there are now available so-called "inductions disks" that will allow non-ferrous cookware to be used on an induction element; using such a disk loses many of the advantages of induction--from high efficiency to no waste heat--but those who want or need, say, a glass/pyrex or ceramic pot for some special use, it is possible to use it on an induction cooktop with such a disk.)

On the horizon is newer technology that will apparently work with any metal cooking vessel, including copper and aluminum, but that technology--though already being used in a few units of Japanese manufacture--is probably quite a few years away from maturity and from inclusion in most induction cookers. If you are interested in a new cooktop, it is, in our judgement, not worth waiting for that technology.

(The trick seems to be using a significantly high-frequency field, which is able to induce a current in any metal; ceramic and glass, however, would still be out of the running for cookware even when this new technology arrives--if it ever does.)

There is also now the first of the new generation of "zoneless" induction cooktops. These essentially make the entire surface of the unit into a cooking area: sensors under the glass detect not only the presence of a pot or pan or whatever, but its size and placement--and then energize only those mini-elements directly under the cooking vessel. You can thus put any size or shape of vessel--from a small, traditional round pot to a gigantic griddle or grill--down anywhere, in any alignment, and the unit will heat it, and only it (or, of course, seveal "its", as may be).

Quoting AEG's brochure: "The hob senses the size of the pan and only heats the exact area covered by the pan. The Maxi-sense range [uses] ‘flexible sections’ to create an all-over cooking surface. Pans can be placed anywhere on the hob as long as the section marker is covered, eliminating the restriction of traditional specific zones [ = elements]. It does not matter how many pans you have or what size they are, whether it is a fish kettle, a small milk pan, or tagine . . . ."

This technology has only been around since about 2006, and in fairness it must be said that early reports on the prototypes were not all that one might have hoped for; De Dietrich, which is to say the Fagor Group, led then, but the prototype as distributed for testing had problems remembering where things were if they were moved about any, and also with uniform heating. Presumably, the engineers learned from what they heard, because such units are now in production and available (sort of--see the note below). We see, though, that Electrolux is into this technology in a substantial way in some of their induction lines, such as AEG. De Dietrich calls it "Continuum", AEG calls it "Maxi-sense" (as seen at the left). One supposes that soon everyone will have it; we feel it is clearly the future of induction, which in a way is to say the future of cooking, for it won't be so long now before gas for cooking is looked back at in the same way we today look back on coal and wood.

The only lines we know of with this technology are Fagor's De Dietrich--its premium, "upmarket" line--and Electrolux's AEG, neither of which is regularly distributed in North America; there is, however, one distributor in Canada--who apparently also ships to the U.S.--who handles some parts of the AEG line, parts which just recently expanded from two induction units to three, the new one being one of AEG's "zoneless" types, though one of only 6.9 kW total and three zones (yes, Virginia, even "zoneless" units have zones) and a somewhat strange profile, wide but shallow. We have no pricing or availability data.

There is also now such a thing as an induction oven. (It looks as if the usual heating coil on the base of the oven has been replaced by a ferrous plate, which is energized to heat by embedded induction coils beneath it--so any sort of bakeware will work in it.) Expect to see more such things before long.

Now Let's Take a Closer Look

(In this part, we use a little math--but don't shudder, it's all just arithmetic!)

First, let's define some terms. Energy is a quantity: it's like a gallon of water. In cooking, we aren't really concerned with actual energy--we want to know at what rate a cooking appliance can supply energy. It's like, say, a garden hose: if it can only produce a dribble of water, it doesn't matter to us that if we let it run day and night we could eventually fill many buckets. What we want to know is how forcefully that hose can spray--how many gallons a minute it can put out--because that's what does useful things for us in some reasonable amount of time.

So, in discussing cooking appliances, we normally talk about energy flow rates, which are just like the water flow rates expressed in "gallons a minute"--that is, we want to be able to know at what rate we can pump heat into the cooking process. For gas, energy content (quantity) is traditionally measured in "British Thermal Units" (BTU), and so the flow rate of gas energy is given in BTU/hour. For electricity, energy content is normally measured as "kilowatt-hours" (kWh) and the flow rate is just kilowatts (kW).

(Let's restate that, because it often confuses people, being sort of "upside down". A kilowatt is not a quantity, it's a rate, like "knots" to measure speed at sea--there are no "knots an hour", knots are the speed, and kilowatts are the electrical energy-flow rate. To measure total energy--as, for instance, your electric-supply company does, to know how much to bill you--we multiply the flow rate, kilowatts, by the time the flow ran, hours, to get "kilowatt-hours" of energy. So BTU/hour and kilowatts are both measures of energy flow rates, not of energy itself.)

The energy in gas and the energy in electricity just happen to be measured in different-sized numbers, but they're measuring the same thing. It's like miles vs. kilometers: we can say a place is about 5 kilometers away, or that it's a little over 3 miles away, but the actual distance we'd have to walk or drive is the same. We can easily convert from miles to kilometers if we know how many of one make up the other. Likewise, we can easily convert from BTU/hour to kilowatts (or vice-versa). There are just about 3,400 BTU to a kWh--or, more exactly, about 3,413. (Keep in mind that a kilowatt is 1,000 watts: 1 kW = 1000 W).

Superficially, then, comparing cooking technologies looks easy: can't we just look at the rated kW or BTU/hour of a cooktop, and simply convert one kind of measure to the other to compare them? Nope. The complication is that the various technologies are not all equally effective at converting their energy content into cooking heat; for example, gas delivers little more than a third of its total energy to the actual cooking process, while induction delivers about 85 to 90 percent of its energy.

That means that if we have a gas cooker capable of putting out X BTU/hour, converting that X to kilowatts does not tell the story--because a lot more of that X is wasted energy that doesn't do any cooking than is the case with induction. To truly compare the cooking power of a gas cooker and an induction cooker, we indeed need to first convert one measure to the other, say BTU/hour to kilowatts; but we then need to slice off from each unit's nominal output the amount that does not get used for cooking.

(Think again of garden hoses: if we have two hoses and each is getting, say, 5 gallons a minute pumped into it by the water tap it's screwed onto, are they the same? Not if one has a pinhole leak while the other has a gaping rip. The amount of water that comes out the nozzle to do whatever we need done will differ drastically from one to the other. Induction cooking has a pinhole leak, maybe 10% to 15% of the raw energy it takes being wasted; gas cooking has the whacking great rip in it, the average unit wasting over 60% of the raw energy it consumes.)

So, to see how induction compares to its only real rival, gas, we have to make the following calculation:

BTU/hour = kW x 3413 x E_ind/E_gas

That last term there--E_ind/E_gas--is simply the ratio of the two methods' real efficiencies: E_ind is the energy efficiency of a typical induction cooker and E_gas is the energy efficiency of a typical quality gas cooker.

The snag comes when we try to find reliable figures for those efficiencies. It is remarkable how much misinformation there is (especially on the internet), largely from well-meaning but ignorant sources who do not understand the issues, or are simply repeating what they read elsewhere (from someone else who does not understand the issues). For example, the energy-efficiency values quoted by various induction-cooker makers range from a low of 83% to a high of 90%, while values given for gas cooking run, depending on the source, from 55% down to as little as 30%, nearly a 2:1 ratio.

Fortunately, in the last few years some standardized data from disinterested sources have become available, so we no longer have to rely on figures from parties with an axe to grind. The U.S. Department of Energy has established that the typical efficiency of induction cooktops is 84%, while that of gas cooktops is 40% (more exactly, 39.9%)--figures right in line with the range of claims made for each, and thus quite believable.

Using those values (and sparing you the in-between steps), we can say that gas-cooker BTU/hour figures equivalent to induction-cooker wattages can be reckoned as:

BTU/hour = kW x 7185

It is worth noting that the testing method that established the induction data used, in essence, a slab of ferrous metal as the "vessel". It reliably established what might be called a "baseline" efficiency, and that is why we use it throughout in evaluating energy equivalencies. It remains as a possibility that particular items of induction equipment--and, for that matter, of cookware--may be a bit more or less efficient than the baseline. There are at least plausible reports that some makes, coupled with some items of cookware, can achieve true efficiences close to 90%. On this site, we do not use that value because we do not yet know of any definite, reliable data, but you should keep it clear in your mind that when we discuss the gas heating-power equivalencies of induction units, we are using what should be considered rather conservative numbers; chances are that many induction units are actually somewhat more powerful (in BTU/hour equivalents) than we set forth.

In fact, Panasonic states for several of its units that efficiency is 90%, noting that: Heating-efficiency measurements were taken based on standards of the Japanese Electrical Manufacturers' Association and using a Panasonic standard enamelled iron pot. Also: a University of Hong Kong research product showed induction efficiencies from 83.3% to 87.9%, numbers clearly in line with 84% as a minimum and 90% as possible.

So How Much Power Is What?

Perhaps the most useful way to use that conversion datum is to see what good gas-cooker BTU values are and work back to what induction-cooker kW values would have to be to correspond. But what are good gas-cooker BTU values? Here too, opinions will vary. As a sort of baseline, we can look at what typical mid-line gas ranges look like. As numerous sources report, a typical "ordinary" home gas range will usually have its burners in these power ranges, give or take only a little: a small burner of about 5,000 Btu/hour; two medium-level burners of about 9,000 Btu/hour; and (depending on width, 30 inches or 36 inches) either one or two large burners of anywhere from 12,000 to 16,000 BTU/hour

When one moves from stock home appliances up to the deluxe level (sometimes called "pro", though ironically the warranties for such units expressly forbid commercial use), gas ranges and cooktops naturally become more powerful. On these, burner powers run up to 18,000 BTU/hour or thereabouts (one highly regarded specimen of this class has four 15,000-BTU/hour burners and two 18,000-BTU/hour burners). One expert source remarked of such gear: Most commercial-style home ranges offer 15,000 BTUs per burner, which is perfectly adequate for most at-home cooks. You won't always need all that heat, but if you want to caramelize a bell pepper in seconds, or blacken a redfish like a pro, well, you'll need all the heat you can get. My advice: Go for the big-time BTUs (which, in the tests he was discussing, was that 18,000 BTU/hour level).

So let's summarize by showing representative gas-power levels and their induction-power equivalents (remember, calculated quite conservatively):

• Typical home stove:
  
  • small: 5,000 BTU/hour gas = 0.70 kW induction
  • medium: 9,000 BTU/hour gas = 1.25 kW induction
  • large: 12,000 BTU/hour gas = 1.70 kW induction; or 15,000 BTU/hour gas = 2.10 kW induction
    
    
• Typical "pro style" stove:
  
  • medium: 15,000 BTU/hour gas = 2.10 kW induction
  • large: 18,000 BTU/hour gas = 2.50 kW induction

(Even for wok cooking, the most power-hungry kind there is, experts consider 10,000 BTU/hour good and 12,000 BTU/hour "hot".)

So how do actual real-world, on-the-market induction cooktops stack up against gas?

It's an almost comic mismatch. Sticking to build-in units (as opposed to little free-standing countertop convenience units), it is difficult, perhaps by now impossible, to find a unit with any element having less than 1.2 kW power--which puts the smallest induction element to be found equal to the average "medium" burner on a gas stove. The least-expensive 30-inch (four-element) induction cooktop has:

• a 1.3-kW small element (between 9,000 and 9,500 BTU/hour),
• two elements of 1.85 kW each (well over 13,000 BTU/hour), and
• one element of 2.4 kW (over 17,000 BTU/hour).

The least-expensive 36-inch (five-element) induction cooktop has:

• a 1.2-kW small element (8,500 BTU/hour),
• a medium element of 1.8 kW (13,000 BTU/hour),
• a larger element of 2.2 kW (16,000 BTU/hour),
• and two elements of 2.4 kW (over 17,000 BTU/hour).

The very highest-power gas burner to be found in the residential market is 22,000 BTU/hour, and that's a sort of freak monster, whereas a 3.6-kW and 3.7-kW element--which is around 26,000 BTU/hour of gas!--is found in many induction cooktops. (Moreover, the elements on some induction units can share power with one another, so that if not every element is already in use, a given one can be "boosted" beyond its normal power level, for uses such as bringing a large pot of water to a boil, or pre-heating a fry skillet.)

So, in sum, induction is not "as powerful as gas"--it's miles ahead.

(There is, incidentally, a lesson there: even really serious cooking does not, save for perhaps a few specialty cases, require stupendous amounts of power, and you should not be seduced into choosing between units sheerly on the basis of the maximum available firepower per element. For one thing, most units of the same size have total maximum unit capabilities that are nearly identical: the differences lie in how they distribute that total among the unit's elements, which are invariably four on a 30-inch-wide unit and five on a 36- inch-wide unit. When a pro tells you that really "big-time" power is the equivalent of around 2.5 kW of induction, you should ask yourself whether getting elements with significantly more power than that really should be a major consideration in your decision-making process.)