Look in to the details. For example, my economy 7 night rate is basically the same as my PV export rate (15p ISH). I used to run a diverter to heat water, but it doesn't make economic sense. I instead run the heat pump off peak, with a COP of 2, it's twice as cost effective as a heating with solar diverter. And in the summer, from 7am-9am, I could be generating 1kW or more from my PV, making it even cheaper.
Also, there's a limit to what you can store, and use. Yes, you could put a massive tank in and heat it to 70c. In the summer that just means additional overheating woes.
Finally, I don't see he point in two immersion heaters (just lower) and a legionella cycle is NOT needed.
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