The most common reason a solar or UPS installation disappoints is that it was sized from equipment nameplates rather than measured demand.
Log first
A week of interval metering across the main incomer and key sub-circuits gives you a real load profile, peaks, base load and the shape of the day.
Separate critical from convenient
Not every circuit deserves battery backup. Splitting critical loads onto their own board usually cuts storage cost substantially.
Design for degradation
Panels and batteries age. Systems sized with no headroom fail to meet the brief in year four.