Solar charging is such a good idea, but currently so poorly executed. The Pico is no exception.
The only way to get a decent charge out of it is to have it in direct and bright sunlight all day - forget leaving it in your car or your conservatory, it won't work. Even with a full charge, the internal battery is weak and at full capacity will only give an iPhone about 1/3 of a full charge, so the best way to charge it is either to leave the phone connected and have the charger sat in the sun for the day, or if you're using it for camping to top the device up from your car's 12V socket - but then if you can do that, you're better off with a phone charger for your car anyway.
Devices like this need a more efficient solar panel and a larger capacity battery to be of any real use. These are OK for an emergency charge, but there are other and better means of doing this. This is a prime example of something that's been designed well but built down to minimum cost and it's lost any usefulness it would have had in the process.