@Opaquer
I hope you don’t mind me jumping in here. I apologize in advance for the length of this message.
I’d like to take a moment to share my perspective on hot water systems, as I believe understanding them can be quite important. While there may be different viewpoints, I think it’s beneficial to consider various perspectives before making a decision.
I prefer to stay positive and hope you find this information helpful.
Why Twin Elements? Twin elements are increasingly useful, especially with rising electricity costs. They don’t double the power but rather feature two separate heating elements in the hot water system, one at the top and one at the bottom.
Each element typically has its own power supply, and understanding why is crucial. The top element heats the water first, and only once it reaches a certain temperature does the bottom element activate. Consult with the manufacturer and your electrician for specific advice, and consider having your electrician install two separate power supplies for the hot water system. The top element heats about 50 litres, ensuring you never completely run out of hot water. The bottom element, powered by the second supply, heats the entire tank and can be used for more energy efficient operation, including smart options.
Switching Controlling
As @sparkydave recommended, it’s better to use a contactor rather than a Shelly device directly to switch the load, especially for something as important as hot water, where reliability is key. This approach is more robust, and I fully agree with his advice. You can use a Shelly to control the contactor… a Shelly 1 mini is sufficient since it only needs to handle the small load required to switch the contactor. If you’re planning to expand your system, consider a Shelly 3 or 4 Pro, which allows for additional controls like garden lighting.
For power consumption monitoring, as Dave suggested, consider using a different method, like a Shelly EM 50. Always consult with your electrician to ensure everything is set up correctly and safely.
It’s best to wire the top element to its own power supply, keeping it on 24/7, so you always have 50 litres of hot water available. The bottom element should be wired through a contactor controlled by a Shelly device, allowing you to turn it on and off as needed. There’s no need for an old school auto/manual/off switch…. you’ve got Home Assistant (HA) for that.
You might consider adding a manual push button in the switchboard. This button can control the contactor through the Shelly, acting as a boost if you can’t access HA or if WiFi is down…. It is a handy safeguard. Just wire it to the Shelly and configure it as a press button.
Below are 2 links as a reference.
Contactor
Boost Button
Smart Automation Home Assistant
To maximize savings, you can set up a timer to turn the bottom element on and off when electricity is cheapest. This simple step can save you a lot of money. I use my own blueprint for this: 
Timer. Additionally, I use another blueprint, 
Trigger - Run ON Timer, which boosts the hot water for one hour when needed.
The manual button in the switchboard, when pressed, turns the hot water on until you manually turn it off. However, since I use my timer daily to heat the water, the system resets each time the timer blueprint runs. This prevents the hot water from being accidentally left on, avoiding any surprise bills. This is why using a press button instead of a switch is crucial.
This is why twin elements are so beneficial. The top element ensures you always have 50 litres of hot water for basic needs, while the bottom element handles longer showers. The boost button is there if you run out of hot water or need a safeguard. While twin element systems do cost more… requiring an additional power supply and a more expensive unit…. I believe the benefits are worth the investment.
What Size Hot Water Storage?
To save money, it’s best to heat your water once a day when electricity is cheaper. Ensure you have enough hot water to meet your needs. For example, I opted for a 400-litre twin element system. The size you choose will depend on your household’s requirements
What Power Hot Water Storage?
Most people choose a 3.6 kW system, but I opted for 4.8 kW. Why? To prepare for future solar power integration. You can feed in power ranging from 0.1 amps to 20 amps. Think of it like an electric stove, where you use the available power. This consideration ties into how solar power can be utilized.
Solar Power
Think of your hot water system like a battery. Instead of selling excess solar power back to the grid, you can direct it to your hot water system first. Once the hot water system is sufficiently heated, you can sell any remaining power. Since hot water is a major cost in the home, using your existing hot water system for storage can be more economical than buying a separate battery.
I chose a 400-litre system with a 4.8 kW power rating to store and utilize maximum power. However, consult with your electrician to ensure compatibility with your solar setup, as your solar system might only produce 3.6 kW. In this case, a 3.6 kW system might be more appropriate.
For solar integration, consider solutions like the Fronius Ohmpilot, which directs excess power to the bottom element of your hot water system. The top element runs 24/7, while the bottom element benefits from excess solar energy, saving you money. Although I don’t have solar power yet, I’m preparing my system for future use.
Hope this helps you in some way.
Blacky 
