Capacity and power answer different questions
Usable capacity describes how much energy is available. Power describes how much demand the system can meet at once. Ask for both, along with expected operating limits and round-trip efficiency.
Model the reason for storage
Solar self-consumption, time-of-use charging, peak-demand support and outage backup are different goals. The best size and equipment can change depending on which goal matters most.
Backup is a designed feature
A battery does not automatically power the home during a grid outage. Ask which circuits are supported, what additional gateway or changeover equipment is included and what power limits apply.