Question: how much energy is required to evaporate 1 kg of water?
Answer: 1 liter of water has a mass of 1 kg. If the water starts off at 100 degrees C, then the energy required to evaporate it would be 540,000 calories or 2.3 x 10^6 Joules. If the water begins at a temperature T degrees, then an additional (100 - T) x 10^3 calories would be required to raise its temperature to 100 degrees.