Turning a dumb water heater into a smart hot water battery using excess solar power with off-the-shelf components

[Albert4]

1. Do you think this way of regulating the input voltage of the boiler with the WPC3700 would work with a thermodynamic water heater instead of a thermoelectric old one?
2. Is there any cheaper solution than the WPC3700 to regulate power. I know you can go for DIY with triac and ESP32 but I woule prefer something out of the shelf and secure - but cheaper than the WPC3700