The solar panel has a sweet spot where with a set amount of sun light it gives maximum output so both the volts and amps vary with the sunlight, so in the morning the volts and amp will be less than at midday. If you fix the volts then the panel will start producing later and will not produce as much at midday. As to the immersion a 220 volt immersion heater at 110 volt will still work but with 1/4 of the output, so there is not reason why one element should be connected in the morning as as the day progressed more and more elements are added, the problem would be if an element failed then the voltage would rise to the rest, so it would need some safety system, but if the panel at midday will only produce 230 volt then using elements at that rating would not be a problem even if voltage drops to 50 volt or less. However some way would need to be found to control how many elements are connected and close down if there is an error, and as far as I am aware this does not exist. So at the moment there is a device which measures the panels output and if more than 3 kW is being exported it switches on the immersion heater as in the early systems you were paid for having the solar panels rather than actually measuring what they produced, as far as I am aware the new meters actually measure in both directions so this is not longer as lucrative.