Stackable modular storage is the top pick for balcony PV users planning phased investment or future PV expansion. Each module is 2.5–5kWh; users start with a small number matching current balcony PV capacity, then stack extra modules for expansion later without full equipment replacement. The unified BMS coordinates parallel modules for balanced charge-discharge and avoids single-cell premature degradation. Compact stacked units fit neatly in balcony corners without blocking solar panel light intake, with dustproof and heat dissipation design adapting to variable balcony conditions. Users upgrade capacity step by step, perfectly matching phased balcony PV installation plans and lowering upfront investment pressure.