As was discussed in

load shedding whats needed to keep watching I do need to add in how to calculate the battery pack you need

volts x Ah / watts / dod x 0.85

Volts is the battery. i.e 12, 24 or 48v

Ah is the Ah of the string, i.e 102, 105, 2000

watts is your load, i.e. 1500w

*DOD = depth of discharge. 2 = 50%, 3 = 33.3%, 5 = 20%*
2 is to account for a maximum target of 50% depth of discharge of the batteries

0.85 is the efficiency of the inverter, i.e loss due to converting ac to dc to ac and maintaining float charge in batteries

So in my case:

I have 4 x 12V 102Ah batteries in series and parallel making a total of 24v and 204Ah.

My load is around 500w but can go up to 1500w

24 x 204 / 500 / 2 x 0.85 = 4h10m

**24 x 204 / 750 / 2 x 0.85 = 2h45m**
24 x 204 / 1000 / 2 x 0.85 = 2h05m

24 x 204 / 1500 / 2 x 0.85 = 1h25m

(edit: type the equation into a calculator as written from left to right. Although the order doesn't matter as long as you don't start with one of the dividers, in which case you need to start with 1/

*divider*.....)

So if I am running at 750w I can easily get 2h30m, or one load shedding block. I have yet to break 500w (without the fridge) average during load shedding and this reflects in the battery level indicator after 2h15m load shedding only dropping 25%. I'm about to install a new plug point for the fridge that will be on the inverter side of the db board. This will add 300w when the fridge is actively working and an average of 100w at idle. I have made this decision to preserve the fridge motor rather than as a need to keep the fridge working during load shedding.