Utility rates often vary significantly by time of day and by season, and are typically highest during afternoon hours in the summer when PV production is highest. Because this calculation doesn't take into account the daily or seasonable higher rates, but instead uses a flat rate, the calculated Payback Period is quite conservative (higher) than the actual payback period is likely to be. You may want to obtain an estimated flat rate from your utility for summer daytime electric rates and use that figure in the payback calculations.