When the thermostat switches the immersion heater off - what happens to the electricity produced by the solar panels?
If a battery-less install NOTHING happens, the solar panels just stop making electricity
The photons still hit the panels, still knock electrons across a P-N junction.
PV panels have an I/V curve. At zero volts, all current flows through the wire. At Voc, all electrons leak back through the diode (which has an exponential curve) and turns into heat. At Vmp, most of the current flows through wires, and maximum power is harvested.
The electricity is still being made, it is just going somewhere else.
If you feed DC from PV through a thermostat, it may arc and burn up rather than interrupting current. It could work at way below rated (AC) voltage. For instance, one company sells a "DC" heating element with thermostat. The thermostat is actually meant for 120Vrms AC. Right around 48VDC it fails, might have worked OK at 36V or below.
Safest approach is to use PV to make AC with an inverter, and feed that to heater.
As 12V and Sandals said, PV may or may not be a great way to make heat. But it could be a good use for excess power not needed for charging batteries or other loads.
For me, power consumed by water heater is small part of my total (grid-connected house) so not worth efforts at conservation. PV is cheap, so I just put in more than I need.
What would you start off with to heat/warm this volume of water in the winter (our latitude is 53.4)?
I think relatively little power is lost as heat leaking through insulation.
Just determine input water temperature, output water temperature, volume consumed per day. Look up how much energy that requires. I think it is one calorie per cc (or gram) per degree. Convert to Joules, which is simply watt-seconds. Use an "insolation" website to determine how much sunshine you get any particular month. Then you can determine how much PV panel is required.
