The typical commercial system combined with the current rebates and tax incentives can usually pay for itself in about 3-6 years. That is considering that the typical utility costs escalate about 7% per year which is approximately the number provided by the California Public Utilities Commission.
If you finance your system, you may realize net savings as early as the first year. Instead of paying your monthly utility bill, you'll be making loan payments for your solar PV system. You will effectively be paying less for your loan than you currently pay your utility company. Your exact payback period will depend on your electrical usage, electric rate schedule, and cost of your system.