Rank loads by the consequence of stopping
Separate controls, communications, essential lighting, process temperature control, refrigeration and production that can restart later. A load needing continuous service differs from one needing enough time for a safe shutdown. Safety-critical and regulated systems require the relevant specialist to define their requirements.
Record interruption tolerance as well as power
List operating power, starting demand, required duration and acceptable interruption. Some equipment tolerates a transfer gap; some controls do not. A backup output does not guarantee seamless transfer. Check switching equipment, controls and site test results. Explain how any existing UPS or generator will coordinate.
Test the backup function separately
Ordinary grid-connected PV generally stops supplying the grid side during an outage. Critical-load supply requires suitable islanding, protection and switching. Acceptance should verify grid loss, transfer, sustained operation and reconnection using the agreed loads. A charged battery on a monitoring screen is not a backup test.
Further reading: www.energy.gov
This article supports early project discussions. Final configuration depends on site conditions.