When you use a unit of solar electricity on site, you avoid buying that unit at your full import rate. When you export it, you receive an export rate that is typically far lower. The gap between those two numbers is where the return on a commercial array is made or lost.
Load shape matters more than roof area
A building that runs seven days a week with a flat load will consume almost everything a roof can produce. A five day office with a summer shutdown will export a large share. The same roof, the same panels, and two very different financial outcomes.
How to raise self consumption
Move shiftable load such as laundry, dishwashing, water heating and vehicle charging into the middle of the day. Add storage where the export gap justifies the capital. Consider an east to west array layout, which produces a broader curve that matches a working day better than a single midday peak.
Size to the load, not to the roof
Filling every square metre is satisfying and frequently wrong. Sizing to the daytime base load produces a smaller array, a lower capital cost and a shorter payback. That is the sizing logic behind the assessment tool on this site.