Brochure payback assumes ideal sun and load. Plants should run the math on their own bills before approving capex.
Minimum inputs
Twelve months of bills, daytime load share, and a quote that already includes structure and electrical work.
Common distortions
Ignoring cleaning and O&M, assuming nameplate yield every day, or forgetting install downtime windows.
Use payback with other goals
Pair it with power quality, production continuity, and phase-2 expansion. A pretty payback on incomplete scope is not a decision.