How To Convert DC Power To Solar Power?

I have a sensor outside that we use 6 AA batteries to run (9 volts total). anyone know how I can convert it to solar power, and charge a battery to keep it going 24-7. I know little about electronics, but not when it comes to solar, and trying to charge, and use power at same time.
THIS UNIT NEEDS TO SAY ON 24/7 !!!!!!!!!! (even at night)