Re: Storing excess solar energy
I've got an Immersun 2 installed (with a 4kW solar PV system). and am very happy with it.
It monitors the grid power usage/feed-in (earlier versions were basically a fancy timer).
If you are producing more power than the house needs, AND if the immersion tank is not up to temp, then it pushes the excess power into the tank rather than the grid.
It controls the voltage to the element, so the watts is flexible, and tends to be anywhere between 100W, to 2kW, depending on time of day, weather conditions etc.
While the water is warming up, you have a net of zero Watts on the grid power. (The Immersun 2 will not take power from the grid, not unless you manually override it anyway).
Once the water it up to temp, the excess power is all pushed back onto the grid again as normal, until the thermostat trips in again, and so on.
So essentially, free hot water....