If you only have one 5 watt solar panel, you do not need a solar regulator. If you have a total of 10 watts or more (10W, 15W, 20W, 30W, up to 80W) of solar panels then you must use one solar regulator in your system. The solar regulator keeps the panels from overcharging your battery during the months with the maximum number of sunny days, or when your geographic region is getting above-average number of daily sunlight hours.