A solar panel from a reputable manufacturer typically will put out the power it is rated for, but only in "optimal" conditions. Optimal means that the sun is perpendicular to the panel and high in the sky. This is mid-day, summer sunlight. In the real world, panels are often not perpendicular to the sun. They are often covered with a layer of dust. Both of these things reduce the power seen at the end of the cable.  In our camper, the cable from the panels is 30 ft. long in total. This is not optimal, so reduces power available to the battery a few percent.  In spring sun, in Denver, our 100 watt panel is typically putting out wattage in the mid eighties. It is almost never at an optimal angle to the sun.  For conservative planning purposes, it is a good idea to assume that a 100 watt panel will put out 70 watts.