Determination of the Energetic Efficiency of an Insulated Water Kettle Compared to a Non-Insulated Kettle

In this experiment, the thermal efficiency of a conventional kettle using an insulation hood developed and 3D-printed by me with a 20 mm Armaflex insulation is investigated. First, water is heated from room temperature (16 °C) to boiling (100 °C), then allowed to cool over defined time intervals. The temperature profiles and the theoretical energy demand required to reheat the water are calculated for both the non-insulated and the insulated kettle. Results show that the insulated kettle requires significantly less energy for reheating, especially during the first 60 minutes after boiling. The percentage energy saving in that period ranges between approximately 34 % and 46 %. Over longer cooling periods, the difference in temperature curves decreases, but even after several hours a measurable benefit remains for the insulated kettle.

Null Hypothesis: The insulation¹[an external layer that reduces heat losses] of the kettle has no influence on the energy demand for heating and reheating water.

1. Introduction

Heat losses²[the release of thermal energy to the surroundings, causing the system’s temperature to drop] are a key factor in many areas of energy technology when considering the efficiency of heating processes³[describes the ratio between the energy actually used for raising the temperature and the total energy expended]. When heating water—whether for cooking or other household tasks—some of the supplied energy is lost to the environment. Reducing these losses can be achieved through suitable insulation solutions. The purpose of this experiment is to determine the effect of a 20 mm insulating layer on the energy required to heat 1 liter of water. In addition, it examines how strongly the insulation influences the cooling rate and thus the energy demand for reheating.

2. Materials and Methods

Further details on the design, structure and the link to the .stl files for reprinting yourself can be found here:

Thermal Insulation of IKEA Metallisk

2. Materials and Methods

1. Experimental Materials
   • Two identical kettles (IKEA “Metallisk”)
   • 20 mm Armaflex insulating material
   • 1 liter of tap water for each kettle
   • A thermometer with sufficient accuracy (±1 K)
   • A stopwatch to measure heating time
2. Setup
   • One kettle remains non-insulated at first.
     • Heat 1 liter of water to boiling and record the time.
     • Measure the cooling rate after the kettle is switched off.
   • Then wrap the kettle completely with a 20 mm Armaflex layer.
     • Repeat the same experiments previously carried out on the non-insulated kettle.
     • 3D-Printing and .STL-Files are here.
3. Procedure
   1. Initial Heating
      • In both kettles, 1 liter of water (16 °C) was heated to boiling (100 °C).
      • The time taken to reach 100 °C was recorded:
        • Non-insulated: 4 min 46 s
        • Insulated: 4 min 16 s
   2. Switching Off & Cooling
      • Immediately after the water reached boiling, power was turned off.
      • The water temperature in both kettles was recorded at defined time intervals (10, 20, 30, 40, 50, 60, 90, 120, 330, 480 minutes).
   3. Theoretical Calculations
      • The specific heat capacity⁴[the energy required to raise the temperature of 1 g of a substance by 1 K] of water was assumed to be

was used as the value for water.

• The energy demand Q for heating or reheating the water is given by the formula

where m is the mass (1 kg) of water, c is the specific heat capacity⁵[energy required to heat 1 g of water by 1 K], and ΔT is the change in temperature. We use c = 4.2 J / (g · K) for water.

4. Determining the Energy Savings:

• At each measurement point, the current water temperature T_reading is recorded.
• The amount of energy Q required to reheat the water back to 100 °C is computed using

• The percentage energy saving due to insulation is found by comparing the difference in energy demand between the two kettles:

3. Results

1. Required Energy for Initial Heating
   • Theoretically (neglecting radiant or convective losses), the energy to heat 1 kg of water from 16 °C to 100 °C is:

• In practice, the insulation already affected the heating time:
  • Non-insulated: 4 min 46 s
  • Insulated: 4 min 16 s

2. Cooling Curves

• The following table shows the measured temperature in both kettles at defined intervals.

Time (min)	Non-insulated (°C)	Insulated (°C)
10	87	93
20	80	89
30	74	85
40	68	81
50	63	77
60	59	73
90	50	66
120	42	60
330	21	33
480	17	24

• Time (min): Time elapsed since the kettle was brought to a boil
• Non-insulated (°C): Water temperature in the non-insulated kettle
• Insulated (°C): Water temperature in the insulated kettle

3. Theoretical Energy Required for Reheating

• The following tables show the energy Q needed to heat the water back to 100 °C, based on the formula

Values in Joules were converted to kilojoules (kJ) and watt-hours (Wh).

Non-Insulated Kettle:

Time (min)	Water Temp. (°C)	ΔT (K)	Q (kJ)	Q (Wh)
10	87	13	54.6	15.17
20	80	20	84.0	23.3
30	74	26	109.2	30.3
40	68	32	134.4	37.3
50	63	37	155.4	43.2
60	59	41	172.2	47.8
90	50	50	210.0	58.3
120	42	58	243.6	67.7
330	21	79	331.8	92.2
480	17	83	348.6	96.8

Insulated Kettle:

Time (min)	Water Temp. (°C)	ΔT (K)	Q (kJ)	Q (Wh)
10	93	7	29.4	8.2
20	89	11	46.2	12.8
30	85	15	63.0	17.5
40	81	19	79.8	22.2
50	77	23	96.6	26.8
60	73	27	113.4	31.5
90	66	34	142.8	39.7
120	60	40	168.0	46.7
330	33	67	281.4	78.2
480	24	76	319.2	88.7

• Time (min): Time elapsed since the kettle was last brought to boil
• Water Temp. (°C): The current water temperature
• ΔT (K): The temperature difference between 100 °C and the measured value
• Q (kJ): The energy in kilojoules needed to bring the water to boiling again
• Q (Wh): The energy in watt-hours needed to bring the water to boiling again

4. Percentage Energy Savings

• The relative savings are determined by the difference in the required energy between the two kettles.

Percentage Energy Savings (Insulated vs. Non-Insulated):

Time (min)	Qnon-insulated (kJ)	Qinsulated (kJ)	Savings (kJ)	Savings (%)
10	54.6	29.4	25.2	46.15 %
20	84.0	46.2	37.8	45.00 %
30	109.2	63.0	46.2	42.31 %
40	134.4	79.8	54.6	40.63 %
50	155.4	96.6	58.8	37.86 %
60	172.2	113.4	58.8	34.15 %
90	210.0	142.8	67.2	32.00 %
120	243.6	168.0	75.6	31.05 %
330	331.8	281.4	50.4	15.19 %
480	348.6	319.2	29.4	8.43 %

• Time (min): Time since the kettle was last boiled
• Q_{non-insulated} (kJ): Energy needed to re-boil the non-insulated kettle
• Q_insulated (kJ): Energy needed to re-boil the insulated kettle
• Savings (kJ): Difference between Q_{non-insulated} and Q_insulated
• Savings (%): Relative savings through insulation

4. Discussion

The experiment clearly demonstrates that a significant energy saving can be achieved by insulating the kettle during the cooling phase. In particular, the insulated kettle retains a substantially higher average water temperature over the first 120 minutes, meaning that—while both kettles require additional energy for reheating—the insulated kettle consistently needs less (Q_insulated is always below Q_{non-insulated}).

Physical Explanation

• Heat dissipation to the surroundings mostly occurs via convection⁶[heat transfer in moving fluids], conduction⁷[transport of energy through molecular collisions] through the kettle walls, and radiation⁸[emission of energy in the form of infrared radiation].
• Armaflex reduces the thermal conductivity⁹[a material’s ability to transfer heat via molecular or phonon interactions] between the hot water inside and the ambient air, which lowers the temperature gradient¹⁰[the difference in temperature per unit distance] at the kettle surface. Consequently, less heat escapes.
• Right after switching off, the temperature difference to the surrounding air is greatest, so the insulation’s effect is particularly pronounced (up to about 46 % savings after 10 minutes).
• Over time, temperatures in both kettles approach each other because all thermal storage reserves¹¹[the capacity of a body or material to absorb and retain heat over a period of time] are gradually released to the environment. Nevertheless, after several hours there is still an observable savings of around 8 %.

Practical Relevance
For frequent use of hot water in short intervals (e.g., tea making or cooking), thermal insulation can noticeably reduce the energy required for repeated heating. Even a 20 mm thick Armaflex layer demonstrates considerable savings potential. For even greater improvements, specialized insulating materials or vacuum panels could be considered.

Because no chemical reactions are performed here, no reaction-based observations are necessary. The observed effects rely purely on the physics of heat transfer. The outcome: A clearly demonstrable reduction in energy needed for repeated heating, conclusively disproving the null hypothesis.

• 1
  [an external layer that reduces heat losses]
• 2
  [the release of thermal energy to the surroundings, causing the system’s temperature to drop]
• 3
  [describes the ratio between the energy actually used for raising the temperature and the total energy expended]
• 4
  [the energy required to raise the temperature of 1 g of a substance by 1 K]
• 5
  [energy required to heat 1 g of water by 1 K]
• 6
  [heat transfer in moving fluids]
• 7
  [transport of energy through molecular collisions]
• 8
  [emission of energy in the form of infrared radiation]
• 9
  [a material’s ability to transfer heat via molecular or phonon interactions]
• 10
  [the difference in temperature per unit distance]
• 11
  [the capacity of a body or material to absorb and retain heat over a period of time]