The raintank is a machine that will build up it's internal buffer with water when it's placed outside in the rain.
As of 2.2 the raintank does not require energy to bottle water.
I. Liquid Tank
Water tank filled by rain water.
II. Liquid Container Slot
Place a Liquid Container (such as a Wax Capsule) in here to have it filled from the contents of the Liquid Tank (I). Filled containers go to III.
III. Filled Container Slot
Filled containers placed in II end here.