Home Depot offers the Coleman 30-amp Digital Solar Charge Controller, model no. 68032, for $68.31 with free shipping. (Amazon charges the same, although stock may be limited.) That's the lowest total price we could find by $15. It's designed to maintain and prevent overcharging and discharging of your 12-volt solar panel batteries. It features an LCD digital meter and handles up to 30 amps of array current and 450 watts of solar power.