Here is a basic overview of the most common financial structures for commercial solar.
ROOF LEASE / ROOFTOP RENTAL
BUILDING OWNER MAKES MONEY, TENANTS SAVE ABOUT 5-10%
A 3rd-party solar company leases the rooftop or other land and pays the building owner rent (all roofs are different, but assume $0.25 to $0.75 SF / year on average). The key requirement is that the owner needs the tenants (AKA off-takers) to agree to buy the electricity produced. This is accomplished via a lease amendment and/or a power purchase agreement between the system owner and the tenant. In some cases like an office building, the building owner would be obligated buy the electricity but then get reimbursed via monthly CAM charges. In my experience, lenders love the commercial roof lease model because it is something they already understand. They view the lease like any other lease and they add the income to the rent roll and update your ratios. Some lenders liken the roof lease to a cell tower lease. The roof lessee maintains the system and the roof in this model.
PURCHASE (CASH OR LOAN)
BUILDING OWNER TAKES MORE RISK BUT CAN MAKE MORE MONEY OR PASS BIGGER SAVINGS ON TO THEIR TENANTS.
The owner can pay cash for the system or obtain a term loan, typically for about 30% down over 7-10 years (1.25x DSCR min). The owner reaps all of the tax benefits. The solar power produced by the owner comes at no cost to the owner, other than the established loan payment. The owner can contract with 3rd party vendors to handle the billing. The owner can sell electricity at a slight discount to the utility and make a profit on the difference between the electricity sold and their monthly loan payment. Downsides are that the owner is responsible for maintenance of the system and lenders may not give the owner credit for the solar income. There is also 100% C-PACE financing available which has more attractive interest rates but the loan is secured by a property tax lien on your property.
POWER PURCHASE AGREEMENT (PPA)
BUILDING EXPENSES ARE DECREASED, POWER COSTS ARE PREDICTABLE.
Like a lease agreement, a 3rd-party solar company owns the system, therefore you are not responsible for the maintenance of the system. Rather than paying a flat lease rate, your payment will be based upon energy production at an agreed upon rate and therefore your payment will fluctuate throughout the year. In theory, the agreed upon PPA rate will be less than the utility rate. The building can lock in their electrical costs for the next 20+ years and not worry about rising electricity costs. Only the owner of the system can sell power to the end user, so there is no opportunity for the building owner to make a profit on the sale of electricity, but they can save money on their building expenses. One downside is that the building owner is obligated to buy the electricity produced by the system when the property is vacant. There is also a very attractive PPA option with an even cheaper rate if the system is secured by C-PACE financing.