20 w
15W wireless charging represents the power charged into the phone, calculated according to 80% efficiency,
The charger power you need to use is greater than 18.75W, but the mobile phone you may be biased, the efficiency may be less than 80%, it is recommended to match a charger of more than 20W.
The charging power of the charger is less than 20 watts can also be charged, but the charging time will be different. So 15w wireless charging can be used to charge a 20w phone.