The Physics of Boiling Water

Heating water is a textbook thermodynamics problem. The energy required to raise the temperature is Q = m × c × ΔT, where m is the mass (1 liter of water = 1 kg), c is the specific heat of water (4,186 J per kg per °C), and ΔT is the temperature rise. Divide by the heater's effective power — rated watts multiplied by efficiency — to get the time.

Why Efficiency Matters

An electric kettle rated at 1,500 W rarely delivers all 1,500 W to the water: heat escapes through the walls, and the heating element itself is not 100% efficient. Using 85% efficiency (a typical value) adds roughly 18% to the theoretical time. Adjust the number if your kettle is old or uninsulated.

Practical Tips

Boil only what you need — heating 0.5 L instead of 1.5 L cuts both time and energy by two-thirds. Use hot tap water to start, which raises the start temperature and reduces ΔT. And remember altitude: water boils below 100°C at high elevations, so less energy is needed there.