that's not a waste of energy, but i bet there was also other habit that is: unless you want to specifically evaporate water, things will get boiled just the same on low or high heat. (heating up to boiling point is most economical using high power) there's zero reason to keep thing boiling on high heat then add water. also, using hot tap water. water heater is much better at heating water than open gas flame, yet i see people insisting on heating entire pots and kettles of cold tap water