The NexStar GT or GTL can be powered in three different ways. Each has its own benefits and drawbacks.
The easiest way is to use the built-in battery compartment. Inside are two holders that fit 8 AA-size batteries. Benefits: no cords to get in your way and no accessories to buy. Drawbacks: batteries last only 45 minutes to three hours in use, possibly having to be replaced right in the middle of your observing session; cost of new batteries and disposal of the used batteries.
Another power source is an AC adapter. For the GT or GTL, Celestron recommends one giving a 12 volt DC output at 2.5 amps, tip positive. Simply plug in the transformer to the wall and the DC plug into the base of the mount. Benefits: continuous, reliable power. Drawbacks: you must be near a source of AC power, you must deal with cordwrap and the hazard of tripping over power cords.
The third power source is a 12-volt DC battery. They are widely available because they are standard for use in cars, boats, etc. Benefits include portability and power to run your scope and accessories as long as all night or several nights. Drawbacks include weight and again the inconvenience of the cord running from the battery to the scope. Celestron’s PowerTank is a specialized DC battery power source that has 12-volt and other voltage outputs, plus conveniences like lights and even a radio.
NOTE: You can plug in AC power or a 12-volt battery to the mount without removing the internal batteries. It is designed to safely cut out the internal power and only use the externally-supplied source. Remove the batteries when the scope is not used for a long time or when using an AC supply or 12-volt battery to prevent corrosion damage to the battery compartment and holders.