It does not change polarity because the plug only fits in one way, none of the six plugs worked for my 5.5mm x 2.5mm power supply, which is very common and should have been included. Also, it is definitely not 30W, it is much less, 9.1W - it barely outputs 1A at 9V and 1.27A at 7.2V, which is 9.1W and not even close to 30W. Furthermore, the cable is around 60 centimeters long, not even 1 meter. After all of this, the price of 10 euros seems extremely excessive for what it offers and it is definitely not what it claims to be. It is a cheap Chinese product with misleading packaging that does not deliver even a third of what it is supposed to offer and is very expensive.