Technical investigation

Measured efficiency of an induction hob and an electric kettle heating water

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The efficiency of this portable induction hob at converting electrical energy into heat in water in this enamel pan was measured as approximately 79%.

This article reports my empirical measurements to estimate the efficiency of a portable induction hob and of an electric kettle heating water to a temperature below boiling point.

A known amount of water was heated for 100 seconds and its temperature change was measured. The electrical power consumed was measured using a plug-in power meter.

The induction hob and the saucepan used are shown in Figure 1 and the kettle is shown in Figure 2. The resulting measurements are shown in Table 1.

The efficiency of this kettle at converting electrical energy into heat in water below boiling point was measured as approximately 84% when the water was left in the kettle for 30 seconds after turning the heating element off.

The figure for electrical power is an estimate of the average, because neither the induction hob nor the kettle consumed exactly constant power.

The power reading for the kettle was 2.142 kW near the start of heating and had fallen to 2.122 kW after about 70 seconds. In general it declined slightly as heating proceeded. (A decline is consistent with a resistive heating element increasing its resistance with increasing temperature).

Power readings for the induction hob were from 2.032 kW to 2.042 kW.

Table 1: Measured values heating water with electrical power applied for 100 seconds.
Measurement Kettle Induction hob
Water mass 710g 700g
Initial temperature 20.2 °C 19.2 °C
Final temperature 80.6 °C 74.2 °C
Electrical power 2.13 kW 2.04 kW

Table 2 shows values calculated from these measurements, including the overall efficiency. The heat energy in the water was calculated using 4.18 kJ/kg as the specific heat of water. The electrical energy was calculated by multiplying the power reading from the power meter by the time the power was applied (100 seconds).

Table 2: Calculated values in heating water with an electric kettle and with a portable induction hob.
Calculation Kettle Induction hob
Temperature change 60.4 °C 55 °C
Heat energy 179 kJ 161 kJ
Electrical energy 213 kJ 204 kJ
Efficiency 84 % 79 %

Detailed procedure

Both the kettle and the induction hob stood unused for hours before they were used for this experimental heating.

The induction heater was used on its highest power setting, a nominal 2000 watts. In both cases after 100 seconds, the power was switched off. In the case of the kettle, the water was then left for a further 30 seconds to absorb more heat from the kettle's base. (The heat reservoir in this kettle's base was investigated in a previous article Illustrating two causes of inefficiency in electric kettles sold in the 2020s.)

In both cases, after this heating period the water was poured quickly into a vacuum flask, sealed and shaken. In the case of the saucepan, a plastic funnel was used to allow faster pouring into the flask. The flask was then re-opened to make the measurement of the water's final uniform temperature.

Temperature was measured with a K-type thermocouple.

Discussion

The calculated efficiency, meaning the proportion of electrical energy used that was converted to heat in the water, was approximately 84% for the electric kettle and approximately 79% for the portable induction hob with the enamel pan.

Both these figures may be under-estimates because no allowance has been made for heat loss that occurs in transferring the water to the flask or to the thermal capacity of the flask. Of course there are also many other potential sources of inaccuracy including the scales used to weigh the water, the thermometer and the electrical power meter. No attempt has been made to quantify these here.

Also it may be possible to get more heat from the kettle by leaving the water for a different length of time after turning the heating element off. The choice of 30 seconds is arbitrary and further work would be needed to optimise this.

For the kettle, this experiment was designed to mitigate or avoid the causes of inefficiency in this kettle highlighted in a previous article Illustrating two causes of inefficiency in electric kettles sold in the 2020s. Here, no heat was wasted by its delayed automatic cut-out, since the cut-out was not used. No heat was wasted vaporising water, since the water did not reach boiling point. The loss of the heat left in the kettle's base was reduced by leaving the water in the kettle for a defined period after the heating element had been turned off.

Under these conditions measurements show that the electric kettle was more efficient than the induction hob.

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