I just put them on the windowsill and let the sun shine on them to provide free energy to my power bank. I bought one for a friend and one for me. I intend to travel in the car and I just put it on the dashboard and it recharges for free. The quality is exceptional. The device has been charged to the sun, so it is difficult to estimate how long it will take to recharge in the sun, but you benefit from a free charge, which I highly recommend!!