We have an santorin with no generator but have 1000 Watts of solar. 400 of those watts are shaded - 200 is behind the main mast flexible panels and 200 is on the new hard top bimini (room for more) The bulk of the power comes from 600 Watts on the arch.
Here is what we did.
1. Added the Victron 712 Battery Monitor that is bluetooth and replaced the Amel Shunt
2. Changed out the AC heating element for a 600 Watt DC element (300 Watt can be used as well)
3. Added a Victron Battery Protect - this serves as the relay and what powers the heating element. - ie the heating element positive cable is connected to the battery protect .
(BPR065022000 Victron Energy BPR065022000 Smart BatteryProtect with Bluetooth 12/24V - 65A) - $59 bucks from pkys
4. Disconnected the thermostat wire from the heating element and attached it directly to the relay.
Here is how it works - The BMV has a relay setting. This is set to Default mode but i changed the settings to Invert. The Low SOC is set to 90 and 96. What this means is that the relay will be powered when the SOC is anything above 90%. When it gets to 90% it will turn off and will not turn back on again until the SOC is 96% (these can be changed) - This send a signal to the battery protect to "turn on" thus sending power to the heating element. The battery protect has a shutdown setting of 12.4 volts (i can change that to really anything but this was set to be an absolule failsafe to never let the batteries go below 12.4). It turns back on when the batteries are 12.75. I have never seen 12.4 from my bank. The tempature sensor is set to what ever the factory setting is...i imagine it is 105 degrees (torried marine water heater).
So everyday and a couple of times a day depending on how much water is used, my heater will turn on automatically in the mid morning as by that time i have recovered from the nightly discharge and am now above 96%/. The element will draw a full 600 Watts until such time that either thhe SOC is 90% OR the tempature sensor says - he we are good and have plenty of hot water. The first time i rant it it took a good 1.5 to 2 hours to get to that tempature. but my batteries never eached 90% as i'm usually producting 400-500 Watts and only have 600 watt drain so max i see going out during the day is 200-250 watts - (if you use a 300 Watt Element and have plenty of solar you won't go negative but will talke longer) Since i have this it only needs to run for maybe 15 to 20 mins and occastionally longer to keep the water at the tempature set by the tempature sensor.
We now take a hot shower everyday and my 2 year old plays in a warm baby pool almost everyday. - All of this is powered with solar. (and soon to be wind)
If i'm on the hard (which i am right now) it still works just the same and If i plug in heat water just by using the battery charger.
If you have a desire (or don't have one) to not have to run your genset to heat water daily then this is a good way to go provided you have enough solar to make it happen. We have one fridge and one freezer and a 12 volt home built spectra watermaker. All of the settings are controlled from my phone via the victron connect app. I can completley turn off the water heater by accessign the battery protect - there is a setting to disable.
I do not have lithium batteries but my bank is large with 4 - L16 Flooded Wet Cell batteries with 840 Amp hours.