The built-in solar panel powers the Solar Thermo Hygrometer by using energy from the sun or ambient light. It can be used both indoors and outdoors to monitor the temperature, displays the high and low of the day with also a built in calendar. It has upgraded new feature of the hygrometer measuring the humidity as well as a comfort index. The suction cup can be switched from front to back (or back to front) to suit your situation. Like this model? Also check out our indoor/outdoor solar thermo