20 Watt SunJack portable solar charging panel with 2 8000Mah Batteries with dual 2 amp (4 amp) USB ports The 20 Watt SunJack comes with two 8,000mAh batteries, giving you enough power to charge eight iPhones after 5 hours of sun The battery also is able to take in a full 2 amp charge from the solar cells, and discharge a full 2 amps - which allows for very rapid energy storage unlike anything else available Dual 2 Amp ports, which is what makes the SunJack charger the most powerful on the market - the 20W puts out 4A of output! The efficiency ~19%, which is higher than average. Weight 3.1lbs. USA support By Gigawatt, Inc. Placentia CA If it's cloudy, a 5 Watt solar panel isn't going to provide enough charge, but our 20 Watt solar panel will still have plenty of juice to spare (allowing for charging even under indirect sunlight or cloudy conditions).