Solar panels only produce the output in the advertising material if they are aimed directly at the sun in bright cloudless conditions.
Unless you can be sure of maintaining these conditions it would be wise to assume the solar panel can only produce about 25% of its advertised output.
No, its about 5 to 15% in overcast conditions, but its not that simple. Voltage and current are not independent for a PV cell,
so as less light falls on the panel you get less current and less voltage.
In practice this means for powering a 7V circuit you need a panel that can produce at least 9--10V open-circuit in full sun,
or more like 10--11V full sun in order to produce best current at 7V in low light.
Typical PV panels are series-connected arrays, and you get about 0.55V usuable voltage per cell
in sun and 0.45V per cell in bright overcast conditions (with good current). For use with a 12V battery
panels have 36 cells (ie 20V good sun, 16V in poor light - you need about 15V to power a battery charger
circuit and another volt for the blocking diode to prevent the battery discharging through the panel at night).
One solution is to use a power-point tracking circuit (effective a buck-boost converter that compensates
for the light conditions to run the panel at its sweet spot and produce the same output voltage. These
cost a bit more but can be combined with battery charger circuit to good effect.