Induction efficienty

Cooking methods that use flames or hot heating elements have a significantly higher loss to the ambient; induction heating directly heats the pot. Because the induction effect does not directly heat the air around the vessel, induction cooking results in further energy efficiencies. Cooling air is blown through the electronics beneath the surface but is only slightly warm.

The purpose of a cooktop is to prepare food; for example, long periods of simmering may be required. A rational measure of efficiency would be to compare the actual energy input to the cooktop to some theoretical value required to prepare the specified food. Since experiments to make these measurements would be difficult to replicate, energy efficiency measurements that are published concentrate on the ability of a cooktop to transfer energy to a metal test block, which is much easier to measure in a repeatable fashion.

Energy transfer efficiency, as defined by U.S. Department of Energy (DOE), is the percentage of the energy consumed by a cooker that, at the end of a simulated cooking cycle, appears to have been transferred as heat to a standardized aluminum test block.

The DOE test cycle starts with both the block and the cooktop at room temperature: 77 °F ± 9 °F (25 °C ± 5 °C). The cooktop is then switched to maximum heating power. When the test block temperature reaches + 144 °F (+80 °C) above the initial room temperature, the cooktop power is immediately reduced to 25% ± 5% of its maximum power. After 15 minutes of operation at this lower power setting, the cooktop is turned off and the heat energy in the test block is measured.^[3] Efficiency is given by the ratio between energy in the block and input (electric) energy.

Such a test, using a combination of two different power levels, was conceived to mimic real life use. Wasted energy terms such as residual unused heat (retained by solid hot-plates, ceramic or coil at the end of the test), and losses from convection and radiation by hot surfaces (including the ones of the block itself) are simply disregarded and don't contribute to efficiency.

In typical cooking, the energy delivered by the cooker is only partly used to heat the food up to temperature; once that has occurred, all the subsequent energy input is delivered to the air as loss through steam or convection and radiation from the pan sides. Since there is no increase in the food temperature, the DOE test procedure would consider the efficiency substantially zero. Cooking procedures such as reduction of a sauce, braising meat, simmering, and so on are significant uses of a cooker, but efficiency of these practices is not modelled by the DOE test procedure.

In 2013 and 2014 DOE developed and proposed new test procedures for cooking products to allow direct comparison of energy transfer efficiency measurements among induction, electric resistance, and gas cooking tops and ranges. The procedures use a new hybrid test block made of aluminum and stainless steel, so it is suitable for tests on Induction cookers. The proposed rule lists results of real lab tests conducted with the hybrid block. For comparable (large) cooking elements the following efficiencies were measured with ±0.5% repeatability: 70.7% - 73.6% for induction, 71.9% for electric coil, 43.9% for gas. Summarizing the results of several tests, DOE affirms that "induction units have an average efficiency of 72.2%, not significantly higher than the 69.9% efficiency of smooth—electric resistance units, or the 71.2% of electric coil units".^[4] Moreover, DOE reminds that the 84% induction efficiency, cited in previous Technical Support Documents, was not measured by DOE laboratories but just "referenced from an external test study" performed in 1992.^[4]

In addition independent tests conducted by manufacturers,^[5] research laboratories^[1] and other subjects seem to demonstrate that actual induction cooking efficiencies stays usually between 74% and 77% and reach occasionally 81% (although these tests could follow procedures different from that of DOE). These clues indicate that the 84% induction average efficiency reference value should be taken with caution.

Just for comparison and in agreement with DOE findings, cooking with gas has an average energy efficiency of about 40%. It can be raised only by using special pots with fins whose first design and commercialization came years ago,^[6] but that have been recently rediscovered, redesigned in a different way and put again on the market.^[7] So for environmental considerations dealing with induction versus gas, a 40% gas efficiency will be used.

When comparing with gas, the relative cost of electrical and gas energy, and the efficiency of the process by which electricity is generated, affect both overall environmental efficiency^[8] (as explained in more detail below) and cost to the user.