Our usage isn't typically higher in winter, as the car is governing so much of the usage to be fair. The occasional heater in a room pales into insignificance...We recently had a quote for a 5.4 kw package which included a 5.2 kWh battery and associated management package aincluding hot water immersion controller to utilise spare power.
The solar including labour, scaffold, panels and inverter worked out at £6929 . This was Solar edge equipment and Trina panels.
The Battery pack was Give energy and with ancillaries added another £4,100. This was all plus VAT which for domestic should be 5% I believe
Do remember the rule of 75% though. 75% of production is in summer and 75% during the hours 10 -2 GMT
This is important your usage will be far higher in the winter ( for most people ) and protection is at its lowest.
to put things in context last month, may best array managed just over .5 units per KW of installed panels per day. At the height of summer this figure would be 4+units per day
hope this helps
Plan B was to look at just installing a battery unit to charge on low rate, and discharge at high rate. Tesla power wall could be charged at our current low rate of 5p/kw, and discharged into the car once it reverts to full rate of 15p/kw.
Assuming these rates double- the capacity of the battery is still only 13.5kw/day, so we're talking saving £3-4/day, with an £8-9k outlay. The car has a 86kw battery....