Some help here? I don't understand this exam question

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On one sunny day no hot water is used in the house.
The water in the panels reaches a constant temperature even though the
water is still absorbing energy from the Sun.
Explain why the temperature of the water in the panels becomes constant.


Whats the answer and why?

i'm no engineer, but I would say it's because the energy being absorbed from the sun is equal to the energy lost to the surrounding milieu.

Or, perhaps the water has reached it's upper temperature limit; 100 degrees Celsius.

cathylynn is correct. It has reached equilibrium with its environment. The hotter the water is, the greater the temperature difference between it and the surroundings, and the faster it loses heat, so the temp climbs until it is losing heat at the same rate that it is gaining it from the sun.
If it were open, then yes the water could evaporate once it was over 100C, but I would think that in panels, it would remain trapped.

A object is at thermal equilibrium when its temperature is the same throughout the object, and when the thermal energy entering the object is equal in magnitude to the thermal energy leaving the object.