The best circumstances in which to charge your WakaWaka is in direct sunlight and without anything blocking the panel(s).
Filtering the sunlight by a glass window can lower the charge current drastically: the glass blocks part of the UV radiation which in turn leads to a little less radiation on the solar cells and a little less sunlight being transformed into electricity. We recommend charging your WakaWaka direct, in morning sunshine or in the late afternoon sun. Try to turn the solar panels towards the sun during the day.
The further you are from the equator, the longer it will take to fully charge the Power+ via the sun.