It does have to switch to the element when it is very cold so some mornings in winter it will use the element rather then the heatpump although I think that it heats half the tank then switches of until it is warmer and then the heatpump tops it off.
I will say it is worth having a roof over it. Our one was exposed to the weather and the bottom outer colour steel cap started to show some signs of rust where the coating had been knocked off during installation. I put a small clearlight roof over that and the aircon's outside unit and that has stopped the rust from getting any worse. It has also helped to cut back the noise a bit. You can hear it when you are out side on that side of the house. Inside you have to strain to hear it.