There is only a 1N5819 Schottky diode in the circuit to carry power from panel to battery during daytime and prevent any current to leak back during night. The power given to the load ( here a clock ) is regulated by the battery and though the power output of the panel is quite low, the voltage clock receives hardly go above 1.3V which is perfectly ok for my cheap chinese wall clock.
The panel generates around 100mA and 4.5V and simply hangs below the clock facing towards the window to get as much sunlight as possible. During daytime, the panel gets 2 hours of direct sunlight and some indirect light so it suffices for the very little amounf of power consumed by the clock. A solar panel with similar power output should do the job but if it doesn't you can simply opt for a larger panel. I've tried this charging method in many circuits with panels with open circuit voltage of 9V and short circuit current of 200mA without problems.