I fitted my solar dump recently as per the facebook 12v group but added in a digital timer (amazon)
Heschen Digital LCD Power Weekly Programmable Timer Relay Switch, CN101A, DC12V 16Amp, SPST, with Waterproof Cover, White : Amazon.co.uk: DIY & Tools
I just set the power controller so the batteries are still getting 2 or 3 amps when its dull, set the upper and lower voltage limits on the smart relay and set the timer to start at 9.30 and off at 2.00 in the afternoon. This is heating enough water for us and is still fully charging the batteries up by 5 pm.
It's not as hot as when on the shoreline but its hot enough for washing the dishes in and having showers etc.
It's a 240v immersion at the top of a vertical surecal calorifier with a 2kw mastervolt inverter. No idea about the immersion length as it supplied plenty of hot water on shorepower so I figured it would do the same on solar and wasn't taking it out to check. I have 1.2kw of panels on the roof.
As we move into summer I'm expecting to increase the run time and hopefully get even better results.