Backup energy and backup power are different
Battery kWh tells you how much energy is available. Inverter/battery kW tells you how much load can run at once. A backup system can have enough stored energy for the night and still trip when a large motor, pump, electric oven or air-conditioner starts.
Essential-load backup
An essential-load design separates circuits such as refrigeration, lighting, communications, selected outlets and sometimes a small pump into a backed-up panel. It can reduce required inverter power and battery capacity while extending runtime during a long outage.
Whole-home backup
Whole-home backup is convenient but still does not mean every appliance can run without limits. Service size, transfer equipment, inverter output, motor starting current, battery discharge limits and load management all matter. Some systems use automated load controls to prevent several large loads from starting together.
Quote the same backup scenario to every installer
Give each installer a list of circuits and appliances you expect to run, their approximate power, motor/surge loads and desired outage duration. That makes quotes comparable. Use the appliance-load calculator, battery-runtime calculator and backup-readiness checklist before requesting installer quotes.