Hello! Welcome to the forum 
Your numbers seem to match with similar style of properties on HeatpumpMonitor.org for December and January. Not bad considering the high water usage.
This will likely be the expansion vessel if it has just one pipe plumbed to it.
The general approach for heatpumps is to adjust the controls to run at the lowest possible flow temperature, without going too low and cycling the heatpump. This usually means adjusting the weather compensation curve (called Water Law on Samsung).
Also, running the heating continuously is usually best for performance and costs, i.e. “low and slow”. [See topic on House Thermal Inertia and Roomstat Setback (some cautionary notes)]
Some heatpumps have some kind of “smart input” that could be wired up with something that knows the price throughout the day. Alternatively, if your controller has any kind of scheduling capability, you could increase the target room temperature during cheap periods, and decrease during peak periods.
PS: fitting a battery to the solar installation would allow you to keep the heating on during the peak periods.