gas is on demand water heating we wouldnt ever run out of hot water like u can with a hot water cylinder(which could be a problem in its self) on gas and rockgas seemed to think we would use around 1 gas bottle a month($112) for water heating and gas hobbs and up to another 1/2 bottle if using the gas fire at nite. that sounds expensive right , but considering we live in an art deco house with no insultaion a heat pump which does nothing, 3kw hotwater cylinder with an uninsulated concrete header tank sitting exposed on the roof; our power bills here are 250 -270 for 3 adults the majority of this is water heating then heatpump . now consider the new house where there will be 5 people to split the rent and utilities with , the house is sunny insulated and not mouldly, every single recessed downlight fitting in the house has energy saving bulbs (so 15w instead of 100w, big differnece when there is over 100 bulbs) also we wont need to run 2 dehumidifyers 24/7 to combat the damp