Two sites with identical hardware can produce very different returns. Four things account for most of the difference.
Business benefits
Lower running costs feed straight into profit. For many organisations there is a second, less visible return in the goodwill and green credentials the system generates.
Property value
Owning a solar system, rather than leasing one, tends to increase the resale value of the property it is installed on.
Technology
A more efficient system offsets a greater share of the monthly electricity bill from the same roof area. Panel and inverter selection is not a detail.
Electric rates
The higher your current tariff, the more each unit you generate is worth. Sites with expensive electricity see the shortest payback.