Normally, you would not have a heating element in the solar hot water tank. You would have it in the hot water heater inside your house.
However, there may be instances where you would want a heating element in the solar tank. For example, you might have a vacation cabin where the solar heater is the only source of hot water. Or you might want to have a thermostatic control to keep the water in the inner tank from freezing during extremely cold, cloudy weather.
A heating element does not come with the solar collector as an installed feature. It must be purchased and installed separately.
There is a single 1" fitting on the left end or bottom of the tank for the heating element which is normally closed with a plug.
The heating element is 3600 watts - 15A @ 240V. Since there is only a single fitting, there is only room for one element. It is adjustable in the range from 10C to 80C (50 F. to 176 F.). The normal setting is about 50C.(122F.)
Since the temperature adjustment portion of the element (thermostat) is exposed outside the tank, the element may not meet local electrical building codes. Check with your electrical inspector. A licensed electrician should be used to install a weatherproof box and outdoor rated wire for this heating element.
If you do not install a heating element, you will want to cover the metal plug with a piece of insulation because it is a point of heat loss.