A landlord benefits greatly from investing in a solar system on their building. It can add tenant loyalty. It can increase tenant loyalty, it increases the value of your building (without raising income taxes), increases your income from that building, and keeps an income coming in from a vacant building. The state and federal governments give increased ratings to buildings with solar and in some cases it is required. SBA 504 loans require solar as a part of their loan commitment.
However, putting solar on a vacant building will not make your solar project cash flow positive. Having a building go vacant before the solar system is paid off will probably cause you some grief if the income from selling the excess generated power to utility company is not enough to make the financing payments.
So, the obvious conclusion is that you want to add solar to a building you own when you know it is going to be occupied for at least 5 years. Most financing is for 5 years, except tax lease financing where the lessor takes the tax credits, which is a minimum of 6 years. Tax lease financing is cash flow positive from day one, so if you put a little away for future payments, you will be covered in case your building goes vacant after the first 5 year period.
You can sell the power to any property that is adjacent by at least a point or across the street, and if you have adequate roof space, then use the same decision making criteria as the rest of this article and increase your income and net worth by investing in more solar. If you install a solar system and your tenant moves out, then look at selling power to a neighbor. This can be done with paperwork, there is no need to run wires to the other building. A case can be made that every large building should have a solar system on it serving adjacent buildings.
Even if you cannot sell power to an adjacent building, you can sell excess power to the utility company. They will pay 25% to 50% as much as a retail customer, but it is a lot better than nothing and a lot better than an empty building will pay rent. Often times it is enough to maintain the costs of a building with the house meter, landscaping, taxes and water bills.
In conclusion, the best time to invest in a solar system on your commercial building is when you sign a new lease. Since everyone sees a positive benefit from solar, both from the quality of the power, the way is shades the building and the overall green outlook, you can use this to help negotiate your new lease. In the case of government tenants, it is a requirement or at least will help you win out over another competing building.