I have one of these set on an Alexa plug (believe it or not) with any excess solar coming in, just program it to switch on! Excess solar then heating water!

This is a 1kw, 11inch immersed length Incoloy Immersion Heater complete with combined (operating & safety cut-out) plug-in thermostat and can be used within both vented and unvented hot water cylinders.
This product is aimed at a low load applications where limited energy is available such as renewable applications in conjunction with Solar panels, Ground Source and Air Source heat pumping and in Marine applications such as such as Calorifiers.
The eCOHeat can be used in all standard domestic water areas including normal use in hard water areas susceptible to limescale. If particularly aggressive water conditions are present or the duty/usage is higher than normal then we would recommend a low watts density or a titanium element version which can be priced on request