Annual escalation
Many leases and power purchase agreements raise what you pay every year by a set percentage, commonly somewhere between one and three percent. Over a twenty-five year term a small annual increase compounds into a substantially larger payment near the end than at the start.
The logic offered is that utility rates rise too, so the arrangement keeps pace. Whether it works out that way depends on what your utility actually does over the same period, which nobody can know in advance.
The step-up after a tax credit
Some solar loans are structured with a lower payment for the first year or so, calculated on the assumption that you will receive a federal tax credit and apply it to the principal. If you do that, the payment continues at roughly the introductory level. If you do not — because you did not have the tax liability to use it, or because the money went elsewhere — the payment steps up to a permanently higher figure.
This is one of the most common surprises we hear about. Whether you were told the introductory payment was conditional is worth checking against the paperwork.
Costs built into the amount financed
The amount you are financing is sometimes larger than the price of the system, because costs connected to arranging the financing can be added to the balance. That does not change your monthly figure by itself, but it does mean the total you repay is higher than the equipment price suggests — and it is often not itemised where you would expect to find it.
Where to find it in your own documents
Look for a schedule of payments, or wording about the payment being adjusted, escalated, or recalculated. On a loan, the disclosure page that came with the financing typically shows the payment amounts and when they change. If your agreement has an exhibit or appendix, the schedule is frequently in there rather than in the main body.