Key use of 40 Watt solar panels is to charge Large AH Batteries. Engineers fit it in Solar Fence Systems with battery backup. Solar panels can then charge the battery during the day to provide a source of electricity at night.
This Panel will give Constant and Full 40 Watt Output During Day as it made from highly effecient Solar cell.
Its Normal or around 20 %efficency will decline after a Long Period of 30 Years.