I have added a 24 liter per minute, instant gas hot water system that runs from a BBQ gas bottle, water pressure and 2 x D batteries. It will heat water to over 60 degrees C, and works a treat. I tried to operate with gravity feed but there was insufficient pressure to allow the heater to turn on. I had to add a 12V DC battery driven pump to create the pressure. This is mounted on the roof and provides approx 27 liters per minute.