Question: how much energy does it take to heat a litre of water?
Answer: One liter of water is about 1 Kg or 1000g, giving Q = m x (delta T) x Specific heat of water = 1000g x 80C x 1 cal/g C= 80,000 cal or 80 Kcals. Technically, 80 kcals that is the amount of heat needed to get the liquid water to 100C and anything extra would go into vaporizing the water into steam.