You may use any power source that provides an input voltage in the range 12-19V DC. Regardless of the input voltage, the power source must provide 65W of power (i.e. a 12V source must provide 5.5A).
The board has a power sense circuit that monitors the input power and will assert the PROCHOT# signal to the CPU to protect against power adapter overheating and/or shutdown. That is, if the input power exceeds 65W, the performance of the processor will be lowered (throttled) to prevent power adapter failure. If the power adapter can produce more than 65W without suffering any overheating or chance of failure, you should disable this power sense circuit. Disabling this circuit is done in BIOS Setup (Visual BIOS). The setting is in the Advanced - Power - Secondary Power settings scene. Uncheck Power Sense to disable this circuit.
Note that 12V is the minimum voltage supported; lower voltages will cause the system to run unreliably or fail. If the input power is from an unreliable source - a car generator or battery, for example, it is recommended that a DC-DC voltage regulator be used. For example, a product like this: DCDC-NUC, 6-48V automotiove power supply for NUC, 12V or 19V output.
Hope this helps,