The solar circuit stops. Under the hottest conditions, the solar hot water tank can reach its maximum temperature (around 55°C to 60°C), known as the “setpoint temperature,” at which point it no longer needs to be heated. When this happens, the solar circuit pump stops working and the water stagnates. It will not circulate behind the photovoltaic cells. This setpoint temperature is a security guarantee for the solar hot water tank.
Overall, the stagnation temperature in the panels will never exceed 80°C (this is the maximum temperature reached by solar cells on a very sunny day).
If the solar hot water tank reaches its setpoint temperature however; it will not continue to heat up in vain.