I've installed one of these recently on my own system to get an idea what it's like before putting to customers. In terms of build quality I have been impressed and it does what it says. I can see from the import meter that it draws everything available that might be exporting and doesn't import anything with only the only clamp. On performance it can quite easily heat our 450l tank on a good day. On poor days it tries it's best but I would question the point of running a 3kw immersion with 50-100volts. On the whole happy it does what it is described to do and expect we would get a reasonable benefit on good generation days. Our set up is a 450l thermal store with log burner and soon to be installed air sourced pump. So on good days I would it would result in is burning less wood and the as pump running less. I would expect because it is proportional it is a better investment than one of the devices that switches on when you are generating more than a set amount running a 1kw immersion as this seems a little hit and miss.
I should add that we have a base load of around 1kW so many others might get a better overall performance as we will be exporting less than many. Saying that I am looking at putting a second 4kW array to the existing 5kW one so it will probably take more advantage of this and prove far more beneficial for us.