Our standard temperature and humidity unit comes with a 25 watt 18 volt flexible solar panel. We chose this one because being flexible it isn't easily damaged as some of the others are. In order to charge a 12 volt battery you need to have an 18 volt solar panel. This panel is a monocrystalline very good quality panel, similar panels sell for $50.00 to $65.0 on the internet. We have several other versions and types of solar panels, but this one is the best.