I've always wished i could figure out how to calculate this. But don't know where to start. scenario: you have to boil a liter of water. if you fill it with cold water from your faucet, it will bill in X amount of time. if you fill the water with hot water from the faucet, it will boil quicker. but to fill the pot with hot water, you will be using heated water from your water heater (pretend water heater and hob are both fueled by natural gas). So I am wondering, if the hot water boils say 4 minutes faster, thats four minutes of time that your gas burner on the hob isn't running, but how much energy is going to be used to replace that hot water you used from the water heater? which method would be cheaper? Im not sure this is even worth calculating, of course its not, but it's always bothered me! what would be some other pros/cons from using hot water or cold water, for cooking purposes?