The SL 3 comes with a 7.5VDC universal power supply, which includes different plugs for the USA, Europe, UK and Australia. The power supply is regulated 7.5 volts DC, 1 amp, using a P6 type barrel plug. If you lose the supplied power supply, you'll need to replace it with one that meets these specifications.
Using the power supply is optional, however it is wise to use one as this will continue to power the Thru channels when USB power isn't present. Without a power supply, the SL 3 will be powered solely by the USB port on your computer via Serato DJ, so if you shut down Serato DJ or your computer or unplug the USB cable to swap over with another DJ, the SL 3 will lose power, cutting all sound.