Calculate the cost to boil a kettle of water.
Boiling water is pure thermodynamics: it takes 4,186 joules to raise one kilogram of water by one degree, so heating a full litre from 15 C to boiling needs about 356 kJ, or 0.099 kWh once the kettle's roughly ninety percent efficiency is allowed for. That is under three cents at typical rates — trivial per boil, but the habit is what costs. Filling a 1.7 litre kettle to make one 250 ml mug wastes about eighty-five percent of the energy, and repeated five times a day that waste alone runs to twenty or thirty dollars a year. Time to boil follows directly from power: a 3 kW kettle delivers 3,000 joules a second, so a litre takes just under two minutes.
Energy to boil water
Energy in joules = litres x 4186 x (100 - starting temperature) / 0.9; kWh = joules / 3,600,000; cost = kWh x rate; time to boil = joules / watts
Proportionally, yes — energy scales linearly with volume. Boiling 0.3 L instead of 1.7 L for a single mug cuts the energy by more than eighty percent, which over a year of several boils a day is a genuine if modest saving.
Yes, and the calculator shows why: starting at 60 C rather than 15 C cuts the energy by more than half, because you only pay for the temperature rise you still need. The taste argument against reboiling is a separate matter.
Usually. Electric kettles are around 90% efficient against roughly 40% for a hob under a pan, so even where gas costs a third of electricity per kWh the kettle is competitive and much faster.