Start with usable energy
Module count is useful only when translated into usable kWh. Estimate daily energy you want to shift or protect during an outage, then compare that target with the manufacturer’s usable-capacity figures.
The inverter may cap the stack
A battery family can support more modules than a specific inverter configuration. Check both documents. For example, the same battery family may allow different minimum and maximum module counts with single-phase and three-phase hybrid inverters.
Power and energy are different
A 15kWh battery does not automatically supply 15kW. Inverter rating, battery current limits and backup architecture determine instantaneous power.
Expansion can have a time window
Some manufacturers restrict how long after commissioning modules may be added or require matched firmware/state-of-health procedures. If future expansion matters, put that requirement in the original design notes.
Check physical and thermal constraints
Stack height, weight, clearances, outdoor rating and operating temperatures can change the practical maximum before the electrical maximum is reached. Use the backup readiness checklist for outage-specific planning.