Optimal energy cost control of a residential grid-interactive PV system is modeled. There is potential of more than 92% cost saving and payback period of less than 14 years. The higher the feed-in tariff (FIT) the lesser the need for a battery bank. Use of battery storage improves the cost effectiveness with low FITs unlike with high FITs. The higher the grid electricity price, the higher the profitability of the system.