I have 10kWh (51.2V, 200Ah) battery pack already.

But, it is not enough to back-up 2-3 days.

So, I have ordered custom battery pack module (total $6,000 include delivery/bus-bar box).

Each battery pack have 5kWh (51.2V, 100Ah) capacity.

Total 30kWh will enough

     2 days     without 0 solar production in winter season

     3 days     with 50% solar production in winter season

     unlimited  with 100% solar production in winter season

My power usage is around 12~16kWh in winter,  

The full solar production is just 10~13kWh in winter, I have to install an extra panels to support full power demand. and planing on this summer

     1.4kWAC panel for grid-tie, expected 2~3kWh extra in winter season

     0.7kWDC panel for battery back-up, expected 1kWh exra in winter season

Basically, no need to worry in spring/fall/summer, solar panel can generate more than a day amount.

just need a night back-up