I just bought a Casio Solar watch. It came out of the box with a medium charge. It recommends leaving in the sun for a few hours to boost the charge up to high, but in the UK in winter, you don’t get many sunny days and I also didn’t fancy leaving it outside. So I tried a little experiment and balanced my LC40 LED flashlight pointing down on to the watch face and left it for a total of about 35-40 minutes. After 15 mins it went to high, then dropped back to medium charge after a few minutes. So I gave it a further 20 mins of light and it went back up to high and has remained there.
I was considering buying a CoolFire Solar Watch Charger for £20, but this torch seems to have done the same for no cost (as I had the torch already).
If anyone else has a solar watch charging problem, get an Anker torch.