PTZ cameras can often be powered by Power-over-Ethernet (PoE), as well as by direct voltage (could be AC or DC or both).
When using PoE, you need to consider the PoE load of your camera and the total number of PoE cameras against the PoE budget of the NVR or PoE switch being used. This is true of any PoE-based installation for WiFi access points, VoIP system, IP intercoms, cameras and so on.
For example, if you take our DMZ150IPW, it is 802.3at/PoE+ capable, so you need to allow for up to 30 Watts of load per camera.
An NVR may only support 802.3af/PoE through its built-in PoE switch, giving you only 15 Watts per camera. Even if it can supply more, as required, for heavier load devices like this PTZ camera, it might not have enough power budget left for the other cameras you wish to use.
Imagine an 8-port PoE supply in an NVR, rated to a total of 120W of PoE load. Add to that two 30W PTZ cameras, and you have 60W of PoE power left for the other six cameras. If each of those 6 other cameras wants 15W, then your cameras may drop in and out, the PTZ cameras work intermittently or not at all, or you could kill the PoE supply or board in the NVR.
In this case, your best bet is to use a standalone PoE+ injector, a PoE+ capable switch (again, be careful of the total PoE load) or take advantage of the direct AC/DC power options.