A solar power system consists of photovoltaic panels with a charge controller. Solar panels are sold by wattage and a 50-75 watt system can usually power one HDX single or multiple antenna reader. The system needs to generate enough power to both charge the battery and power the system during the day. If the panels will be in a valley and not get enough sun then more panels can be added to make up for the deficit.
This USGS publication gives details about building solar power sources.