We know that the energy required to raise the temperature of a substance is the product of that substance's mass, specific heat capacity and the change in temperature. Mathematically:
Q = m * Cp * delta(T)
The Cp of water in Kcal/(kg*K) or Kcal/(kg*C) is 1. (Per unit Celsius and per unit Kelvin are interchangeable because one degree change in Celsius is equal to one degree change in Kelvin)
Using the formula,
Q = 0.5 * 1 * (50-20)
Q = 15 Kcal
