Heating or cooling a substance takes energy, and how much depends on its mass, its specific heat capacity (how resistant it is to temperature change), and how much you want the temperature to change.
Heating or cooling a substance takes energy, and how much depends on its mass, its specific heat capacity (how resistant it is to temperature change), and how much you want the temperature to change. This calculator finds that heat energy in joules — handy for everything from boiling water to material science.
Heating 2 kg of water (c = 4186 J/kg·°C) by 30°C. Q = 2 × 4186 × 30 = 251,160 J (about 251 kJ).
Using Q = mcΔT, where Q is heat energy, m is mass, c is specific heat capacity of the material, and ΔT is the temperature change.
It's the amount of energy needed to raise one unit of mass of a substance by one degree — different materials require very different amounts of energy for the same temperature change.
Water has a much higher specific heat capacity than most metals, meaning it requires significantly more energy to achieve the same temperature increase.