Two possibilities:
With respect to a single Solar PV Panel:
Utilization Factor = efficiency of a solar panel (Energy
captured out of energy available)
With respect to an entire Solar PV System:
Utilization Factor = Ratio of DC in solar panels to AC inverter
rating. Typically a 1.15-1.18 utilization factor is desired. This
accounts for
1. DC line loss from the panel to the inverter
2. Degradation of solar panels over the system lifespan
3. Non-tracking systems use extra DC from panels when the sun is
not direct