1 — Load size
The size of the critical load sets the capacity of the initial installation. Size for the current critical load plus a modest 6–12 months of growth.
Rather than oversizing a single unit, a modular / parallel topology lets you add units later — for capacity and for redundancy.
2 — System reliability
The configuration depends on how much reliability the load needs. A single UPS module with a static bypass can serve loads with lower requirements, drawing on utility or generator power through the bypass.
High-reliability loads need redundant modules and multiple battery banks.
3 — Battery run time
Battery run time is the length of time the UPS can reliably supply the critical load after the input has failed. A typical figure is around 15 minutes — enough for an orderly shutdown.
Batteries are heavy; a large bank may need a structural review of the floor.
4 — Future expansion
Plan the space and the electrical infrastructure for what the site will need later. Installing a smaller module now with provision to expand can be more economical than one large unit bought up front.
5 — Budget
Budget shapes the final design. Cost limits usually mean trading off some functionality or some redundancy — the point of the pre-sales study is to make that trade deliberately, not by accident.