Take twelve months of bills and find the evening consumption, from sunset to the point the house goes quiet. That figure, in kilowatt hours, is what the battery has to cover on a normal day. It is almost always smaller than people expect and almost always different from the size suggested by the solar array.

Then add the backup requirement separately. Ask which circuits must stay live during a cut and for how long: a fridge, a router and some lighting is a very different number from a house with an electric hob and a heat pump. Add those two figures, then add headroom for depth of discharge.

Only at that point does the product matter. A 10 kWh requirement can be met by one wall-mounted unit or by two stacked 5 kWh modules, and the right answer depends on whether the household expects to grow the system later. If it does, stacking costs less over five years even though it costs more today.