In thinking about any 3rd party financed solar project it is helpful to visualize a triangle. One one side is your customer, on another is the investor and the final side is everyone who needs to get paid in order to do the work.
In order for the project to be viable and gather enough momentum to get done there needs to be enough positive benefit that each side of the triangle thinks it is worth doing.
We usually start with the customer. Without a customer there is no project so there needs to be enough value that they are willing to go through the process and agree to a long term contract (lease or PPA), and allow us of their space. Primary value for the customer is usually related to a savings in electricity rates. Additional value includes positive image, hedging future price increases, employee morale, sustainability initiatives and the feeling of doing the right things. But in most cases you need the primary benefit of electricity savings to move the project along.
From there we need to look at the investor. There needs to be enough positive value for the investor. This is almost always ROI for the investor. That after all is the point of investing. Depending on the type of investor and the risk of the project the ROI that is needed varies. This is usually referred to as the "hurdle rate" and tends to be between 5% of the low end to 15% on the high end for a PV project.
Then of course there has to be enough money to build, insure, operate and finance the project.
If you can build the project, meet the customers needs on electricity costs and the investors hurdle rate you have a project that "pencils"
One major mistake that a lot of inexperienced developers make is to focus on one side of the triangle. They sign a contract with a customer that gets the customer where they need to be but it won't work for the investor.
Or they focus on the costs to get it built but don't have a good perspective on the two other sides of the triangle.
Another typical approach is to understand where the investor needs to be and assume that the customer will move to a place that allows for that.
All three sides of the triangle need to be satisfied for the project to be viable.