mikeblackburn

Thanks... have managed to figure it out - have the TH16 attached to the hot water pipe on the geyser - it sends temp info to HA which then nodeRed's the CBI breaker. seems to be working quite well!

@mikeblackburnHome Assistant has a easy integration with Sonoff. You can have a look at this video. There has been some changes with HACS since this video was released, but it should give you a good idea where to start.
I've been using my TH16 on my geyser for almost two years now without a problem. I have a 2kW element in my geyser. 
If you have a smart circuit breaker you can do it as you describe above, but you will have to put in a second power supply for the sonoff TH16. Otherwise, if the circuit breaker switch off, the sonoff will also switch off and you won't be able to see the temperature
 

That won't work if you go into loadshedding with the device on and exit loadshedding outside the desired operation time... (or vice versa)

Not sure how to share the flow but here is the schema

Well he has it connected to Home Assistant. So here is another way to turn it on.
Basically, this automation run every minute. If the conditions (start and end time to be on) are met, then we turn it on.
automation: ## Whole night - alias: "Turn on outside lights and check every minute - whole night" trigger: - platform: time_pattern # You can also match on interval. This will match every x minutes minutes: "/1" condition: condition: or conditions: - condition: sun after: sunset #after_offset: "00:20:00" - condition: sun before: sunrise #before_offset: "-00:20:00" action: - service: light.turn_on entity_id: - light.lights_whole_night In this case I am turning on a group of lights during the start of sunset and end of sunset. 
You can do the same with the geyser. Then you just need another automation to turn it off again afterwards.

This is one way of doing it, there are unlimited ways of doing this in home assistant.