Measured efficiency of a small, novel electric kettle
by Stephen Hewitt | Published
This article reports empirical measurements of the efficiency of a small, novel electric kettle heating a small amount of water to a temperature below boiling point. This kettle was bought in 2026 in Britain from Aldi, the supermarket chain.
The kettle is novel because it is like a saucepan. The lid is the full width of the pan, which allows cooking of vegetables and more, though the manufacturer says it is intended only to boil water and “warm up soup”. The manufacturer says it is rated 509 - 600 W, but I measured its power consumption as 609 W (see below).
The body has only stainless steel in contact with the water, made like a saucepan, of one continuous piece with no joints. The lid has also stainless steel facing the water, with the exception of a detachable seal that looks like silicone. But if the lid is removed before pouring, the water touches only stainless steel. (Figure 1)
Method
The power consumption of the kettle was assumed constant and was measured with the supplier's meter used for billing (Figure 2). A weighed sample of water (approximately a mug full) at room temperature was put into the kettle and the initial temperature of the water was measured in the kettle. It was heated for 180 seconds and its final temperature was measured. All temperature measurements were made with a bare K type thermocouple.
The electrical energy used was taken as the power of the kettle multiplied by the time that it was heating the water. The efficiency was taken as the theoretical amount of energy required to raise the water temperature divided by the electrical energy used.
To measure the final temperature of the water, at the end of heating period the kettle was turned off and the water was poured as quickly as possible into a pre-heated double-walled 500mL food flask and sealed. The flask was then shaken and inverted a few times, and then re-opened and the water temperature immediately measured. The motivation for the flask is to ensure that the heated water is a uniform temperature when it is measured.
The flask was pre-heated by keeping a similar volume of water in it from a trial run of a previous heating and shaking the flask with this hot water and then emptying the flask just before pouring in the sample water.
To measure the power consumption of the kettle, the turns of the wheel on the supplier's electricity meter, shown in Figure 2, were counted over a measured period of time while the kettle heated about 0.8L of water (its maximum), with no other load on the meter. As the photograph shows, it is easily possible to read the position of the wheel to less than one hundredth of a revolution.
However, the meter was evidently not accurate to the nearest division. On one occasion I saw it continue to rotate by several of these divisions with no load and on more than one occasion to rotate several of these divisions backwards with no load. On one occasion it moved backwards from position 0.41 of a revolution to 0.32 over a period of about 5 minutes. In fact several times I came to the meter after using a random amount of electricity and found the wheel at position 0.32, so I suspect it has a favourite position that it likes to return to, forward or backward, when no electricity is being used.
So the motivation for measuring the electricity used over a longer period than the actual heating of a single mug of water, was just this, that the meter does not seem to be precise enough to measure a shorter period.
Results
While the kettle heated 0.8L of water continuously for 8 minutes 48 seconds, the wheel on the electricity meter made 14.89 revolutions, with no other load.
Since the meter face states that there are 166 2/3 (ie 500/3) revolutions in a kW hour, we can say that the measured revolutions represent the following electrical energy:
Energy = 14.89 * 3/500 * 3600 kJ
Since 8 minutes 48 seconds is 528 seconds:
Power = 14.89 * 3/500 * 3600/528 kW, which is about 609 W
This means that during the 180 seconds heating the sample water, electrical energy used was
Electrical energy = 14.89 * 3/500 * 3600/528 * 180 kJ, which is about 109.6 kJ
The sample water weighed 335g and its starting temperature was measured as 22.2 °C in the kettle and after 180 seconds heating, its final temperature in the flask was measured as 85.6 °C.
This is a temperature increase of 63.4 °C or Kelvin and taking the specific heat of water as 4.18 kJ/(kg K), we can say this represents:
Heat energy = 0.335 * 4.18 * 63.4 kJ which is about 88.78 kJ
So efficiency = 88.78/109.6 or about 81%