Using the power generated from a Solar PV system can be expensive if you use one of the commercial devices .Expensive by my definition is anything over GBP150 or US$200.
Most hot water tank heating elements are 3KW, which on a 240V system is about 12A.
An average domestic solar PV system can generate between 3 to 5KW, that is in mega bright sunlight. In real life the mean level on a sunny day on a 4.8KW system like mine is 2.7KW.
So if you connect the 3KW load you end up taking power from the grid as well. 
However, if you only had a 1.5 KW load , (1.5KW immersion heater element ). You can connect it most of the day when the sun is out and generating 1.8KW or more.
My mean usage is about 300w during the day, hence the 1.8KW.
The nice thing about immersion heater elements is they are nearly purely resistive, so you dont need to worry about things like back emf when switching.
You can reduce the power taken from 3KW to 1.5KW by putting a series silicon diode in the feed to the heating element.
I used a Fairchild RHRG75120 75 A, 1200 V, Hyperfast Diode , low cost about £4 each
They are in a flat package and single hole fixing to a heatsink with an insultaing bush kit.
It does not need to be a huge heatsink , it doesn’t get very hot.
The forward voltage drop is about 1V, and at 12A , thats 12W ish
Use a relay or small contactor to switch the power to the heating element, its too much
to use a sonoff or shelly1 .
If you use a relay with a coil voltage of 240V ac, and contacts rated at 30A, cost £10 max
Use a sonoff or shelly switch to control the relay.
Total cost under £25, much better than the commercial unit price
My section of the automation .yaml :-
- alias: Solar water boost on
trigger:
platform: time_pattern
minutes: '/5'
condition:
condition: and
conditions:
- condition: template
value_template: '{{ states.sun.sun.attributes.elevation > 35 }}'
- condition: template
value_template: "{{ states('sensor.solar_power') | float > 1800 }}"
- condition: template
value_template: "{{ states('sensor.hot_water_temp') | float < 53 }}"
action:
service: switch.turn_on
entity_id: switch.solar_hot_water
- alias: Solar water boost off
trigger:
platform: time_pattern
minutes: '/5'
condition:
condition: or
conditions:
- condition: template
value_template: '{{ states.sun.sun.attributes.elevation < 30 }}'
- condition: template
value_template: "{{ states('sensor.solar_power') | float < 1700 }}"
- condition: template
value_template: "{{ states('sensor.hot_water_temp') | float > 55 }}"
action:
service: switch.turn_off
entity_id: switch.solar_hot_water
If you dont have temperature monitoring on the hot water tank just use the normal thermostat
set at about 55C, or however hot you like the water.
In my case I have panels facing east and west and on a sunny day my system is producing enough when the sun is 35 degs above the horizon.
NOTE: in the winter the sun may not get to 35degrees elevation, but unless you are lucky you dont generate a lot then anyway.
If you dont understand what putting a Diode in series means, DO NOT attempt this.
And when you are happy with the results put it in a flame retardent ABS box, so not prying fingers can touch anything thats live.
On mine I also put another relay with the contacts across the diode, so when energised it shorts out the diode, so full power can be applied to the heating element.
But thats only belt and braces approach in case the main fuel used for heating the hot water here, GAS, was to go off line .