Solar panels are rated by the battery voltage they should charge. A "12V" panel will charge a 12V lead-acid accumulator even when cloudy. Open circuit full-sunlight voltages are a lot higher than under load when cloudy voltages. The panel has to drop voltage through the series diode and then provide 14V (fully charged 12V lead acid goes up to nearly 14V).
To get the most power out of it a maximum-power-point (MPP) tracking charge controller would be needed - these dynamically vary the amount of buck or boost in a DC-DC converter to keep the cell at its maximum power point.
If you are only interested in power in full sun then that panel probably should be operated around 15 to 16V.