Good day. In my personal opinion one of the better options that I've seen, is to use one of the Geyser tech controllers Geysertech Pi2R iX Geyser Solar ControllerIs a good option with an input voltage from 90v-325v DC, with 3 panels per controller you will have current flowing to your geyser. The unit can run from pv only with an option of an AC input as well if you need to flick a switch and add power from your battery backup to increase the temperature rapidly. These units I have seen to be efficient in getting the water hot if the sun is shining. In my experience at my house my 150l geyser use 7.5kw to heat the water from cold to 73 degrees, then our whole household shower and we have hot water for dishes. My drawback is on cloudy days I don't let my geyser run the full allotted time in order to get my batteries full. In my installation I've reset the thermostat on my geyser by first turning to 0 and then to maximum. I have relocated my geyser from the ceiling space to outside of the house and from horizontal to vertical and by insulation of the hot water outlet pipe for the portions that you can will also save on your heating bill, I currently still run my geyser off my inverter but would run one of the said controllers when I got my panels sorted. With the said controller the output just go to the standard thermostat terminals and your geyser remain stock standard and when the element fails it is a standard affordable element and not the expensive elements that one find these days from various brands. There is a smaller cheaper version of the controller as well, smaller version input is from 90v - 200v DC if I recall correctly. Hope this helps