Partial power is produced on overcast days, but any shadows falling directly on a solar module will reduce power, regardless of what some advertisements claim. First used 40 years ago for space satellites, modules of that vintage still operate unless the glass has been broken or water has entered the seals. Life expectancy of solar modules is excellent. Most photovoltaic modules come with a 20 to 25 year warranty on power output and may last a lifetime. Overcast days with only half normal brightness give half of normal power.
Some climates allow much more power in summer than winter.
Solar modules can easily be added to your array to increase power as your needs grow. Modules are rated by volts, watts, and amps. The AMP rating is the best indicator of the charging you get and is what you will see on your meter during ideal sun conditions. Siemens residential power modules use quality single crystal cells grown as a cylindrical crystal. The crystal is sliced into round cells.