The answer in detail
Start by identifying services and their dependencies. Alongside servers and storage, switches, routers, firewalls, telephony, management systems and, where applicable, cooling or control components often need a defined power supply. A server that keeps running but cannot reach its storage or network may not meet the protection goal.
Not every load needs the same runtime. Critical core systems can take priority, while less important loads can shut down earlier or remain outside the battery-backed supply. This reduces power and battery requirements.
ERREPI sizes the UPS using a list of loads and dependencies. This establishes what must keep running, what should shut down in an orderly way and what may stop immediately during a power failure.
More on this topic: Server room and network.