When water boils it changes from liquid to a gas. In order to do this it is necessary to add energy (the energy 'frees' the water molecules from the liguid and gives them enough energy to exists separate from othe molecules in a gaseous state.)
Note - Although you are adding heat (energy) the temperature does not change during the vapourisation. Both th eboiling water and the gaseous steam are at 100degC (under normal conditions) The energy is absorbed by the vapourisation process
The amount of energy required to do this (for each Kg) is given by Hv
The units of Hv are J/kg Joules PER kilogram
PER means 'for each' so 1 kg of water takes 2.26x10^6 J to vapourise.
You should see an easy sum to work out how much heat 1.5kg takes.....
