Quote from the user manual:
IMPORTANT NOTE: When solar recharging your Rockpower 1300, take
note of the voltages of the solar panels. Individual solar panels, or solar
panels chained in series in excess of 26V cannot be used with the
Rockpower 1300 and can cause serious damage to the unit.
This means that you must use panels that are a nominal 12 volt. Most 100 watt panels are 12 volt. A 12 volt panel can have an open circuit voltage (VOC) of about 21 volts.