Outlet power from utility grid is constant - constant voltage and constant current according to load. Moreover, grid is capable of providing large amount of power at any time so it can charge a battery or other larger devices faster. But power from solar charger is dependent on many factors. If it is a full sunny day, charging would be fast but not during cloudy days. Similarly, if solar panel would be directed towards sun, charging would be fast. Yet any solar system has maximum power capacity that could be much lower than device being charged. So solar charging takes much longer time than charging from an outlet.