I think DJ only ever used the long, bath, element, doesn't say if its solar thermal or solar PV thats being installed but either way should look at getting the immersion element with the thermostat rod the same length as the element otherwise if the top of the tank is hot, a short, 11" rod thermostat may not close when 1/2 the cylinder is cool/cooler, very important if using a solar diverter to heat all the tank.