You can't. You might as well try to convert dollars to gallons of gasoline. There is a relationship, but that relationship will vary depending on a number of circumstances (location, the passing of time, the grade of gasoline desired, etc.). There is a relationship between KVA and KW, but that relationship will vary depending on a number of circumstances (specifically the amount of resistive load, the amount of inductive load, and the amount of capacitive load).