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Mains Timer

Ideally hot water would be used during peak PV production, however reality has most of the hot water being used in the morning before PV production starts.
This means the mains element heats the water back up to temperature after usage, and the PV boosts it again to an even higher temperature throughout the day.
This would not be an issue, but for the fact that the temperature differential between the hot water and ambient temperature is increased, resulting in an increased rate of temperature reduction of the hot water.

Installing a mains timer that only enables the mains element after PV production finishes should mitigate this issue by using the PV to to heat cooler water, rather than super heat already hot water.

Job Done

Okay I installed the mains timer and set it to switch off at 05:00 and back on again at 03:00. We will see how it goes and update the timings if necessary. 
HWC timer
And here are some results. Notice the reduced heat loss due to the decreased difference between the water temperature and the ambient temperature. 
mains timer graph