The outlet size on a Sanden Heat Pump’s water tank is 3/4 inch (DIN20) with a BSP thread, so be aware you’ll have to use a reducer to get down to the normal 1/2 inch (DIN15) copper pipes. At least, that’s what I was told on the phone. The Sanden specification sheet, which has legible diagrams and writing compared to the installation instructions for an Australian Sanden, says the thread is NPT – but admittedly the model numbers are different (GAU-315EQTE on the clearly Australian installation instructions, GAUS-315EQTD on the American spec sheet). It also gives the dimensions, which when turned into SI units are 756mm H x 883mm W x 362mm D for the compressor and 1490mm H x 678mm W diameter for the 315L tank. For further fun, the installation manual includes a photo of an installed system on the first page – the system is installed too close together, according to the instructions.
However, 3/4″ pipes are able to transport a greater volume of water per unit time. If you’re running warm water down them for bathing, rather than hot for subsequent mixing, greater carrying capacity means better water pressure when running multiple showers, for example. If one was replumbing a whole house, and doing so with pre-mixed warm water, replumbing with 3/4″ copper would be a reasonable thing to do.
Be aware that the cold water inlet has a maximum pressure of 650 kpa.