Solar water heaters are manufactured as pressure and non-pressure models. Pressure model (also called high pressure solar heaters) means, water in the tank are under high pressure, it's equal to the pressure of tap water. Non pressure model (also called low pressure solar heaters) means, water in the tank is under low pressure, and is equal to the gravity of the water. There are some fundamental differences which you should be aware of before deciding which one works best for you and which one you should install.