Virtually nothing in December and January although on a bright but cold day, 3-4kWh is possible.
This maybe all you wanted to know but here is a bit more if you are interested. Heat input comes from solar thermal tubes and any excess PV. A diverter acts to send any spare pv to the immersion rather than export any to the grid. In the winter, the PV goes to an immersion half way up the tank, in summer it goes in at the bottom to increase the storage potential. In winter effectively only the top half of the store is in use with inputs from boiler and excess PV and extracted from the top via the DHW heat exchanger. The tank is allowed to stratify. In summer the whole store is used. Inputs from PV and solar thermal go in at the bottom and a de-stratification pump can optionally be used to maximise the energy capacity by the whole store being at 93 C Summer and winter are simply names for the two modes and not calendar definitions. Typically winter mode is used from November to mid March
The control system is work in progress, but at the moment, the tank stat that controls the boiler is set to 60C and the timer runs the boiler for
20 mins in the morning which gives enough DHW for morning ablutions and odd needs during the day (we are all (3 adults) at home all day normally). If there is any solar input, this adds heat during the day and then at 5pm the boiler runs for 30 mins as needed. If the solar gain has been significant , then the tank stat will turn the boiler off sooner.From March to October, it is rare for the boiler to fire at all for DHW
The cylinder is very well lagged (75mm foam) even so measurements show that it loses 10-15% of its heat capacity over night.
My proposed control system will endeavour to minimise the energy stored overnight when it has been sourced by burning gas. The main input to this will be my "calorimeter". This measures the cylinder temperature at 7 places vertically (the store is 2.2m tall deliberately to allow stratification), as well as the output temperature from the TMV (its control knob is not calibrated) and the cold inlet temperature. From this data, it calculates how much water can be drawn off before the output temperature would fall below the setting of the TMV. The calorimeter is all working but just has a display at the moment. Eventually it will be integrated into closed loop control.
A few pics
Cylinder and sensors:
Control panel (under development):
Calorimeter display:
top row: cylinder temps top(right) bottom (left)
2nd row: Actual in and out temperatures 3rd row: cold temperature (trough hold-slow rise) and output temperature (peak hold-slow fall) bottom row: Processing time mS: calculated volume of useful hot water: software version number.As you might have gathered this has been a bit of a 'project' undertaken to keep me out of mischief since retiring.
There is a bit more of a write up here
Bob