How many watts you need mostly depends on what your phone or device can handle and when or how you want to use the charger. Do you want the fastest charger possible to fast charge your device? In most cases a 10W charger or 15W charger will suffice, as the maximum amount of Watts for fast charge is capped by smartphone manufacturers. For Apple Fast Charge this is 7,5W and for Samsung Fast Charge this is 9W. Make sure your phone actually support fast charge such as Apple Fast Charge or Samsung Fast Charge. If you want to charge your device at night, a 5W charger will also suffice.
The higher the power output of a charger is, the faster your device will be charged, right? Why buy a charger with a 10 Watt (W) output if the same charger is available in a 15W version?
Well, more power does not necessarily mean faster charging. In some cases it may even make more sense to choose a 10W wireless charger over a 15W version.