For Best Results, charge during mid-day hours when sun is directly overhead.  This will provide ultimate efficiency.  The product will still work in early morning or afternoon sunlight, but may not generate full power.  In addition, product works best under clear skies, but can work in overcast conditions to a lesser degree.  If able, try to create a 90 degree angle between the sun and the solar panel face.  A rule of thumb is that if you can create a shadow, you can charge your device.  Simply plug your device's manufacturer approved USB cord into the SunSoaker connector port and enjoy the benefits of free electricity!