ZDNET Expert Urges 20W iPhone Adapter Under 'Plus Five' Charging Rule

Q: Your iPhone charges at half speed. Are you making this common mistake with your power adapter?

Yes. The most common reason an iPhone charges at roughly half speed wirelessly is an underpowered or mismatched wall adapter, often combined with a non-certified charger or cable. Experts cited by ZDNET say many users plug a 15W wireless charger into an older or weaker power brick, which forces the charger to throttle. Wireless charging already loses about 20% to 30% of power as heat and magnetic leakage, so a weak adapter makes the slowdown much worse. Their fix is the “Plus Five” rule: use a wall adapter rated 5W higher than the charger’s maximum output. For a 15W Qi2 or MagSafe setup, that means at least a 20W USB-C adapter. For newer 25W-capable iPhones and chargers, use 30W or more. Certification also matters. Many “MagSafe-compatible” chargers are not actually MagSafe or Qi2 certified and may top out at 7.5W, which is about half the speed of proper 15W charging. Low-quality or uncertified USB-C cables can also reduce power delivery. Another common mistake is sharing one wall adapter across multiple devices. If your charger brick is powering a phone, watch, and earbuds at once, the wattage is split and your iPhone may slow dramatically. Experts recommend dedicating one adapter to the wireless charger. If charging still feels slow, check alignment on the pad, remove thick or metal cases, keep the phone cool, and avoid heavy use while charging. Heat triggers iPhone thermal protection, which can cut charging speed further.

1 articles · Updated · ZDNet · Jun 30

A 20W USB-C wall adapter is the baseline fix ZDNET recommends for slow iPhone wireless charging, following a “Plus Five” rule of supplying 5W more than the charger’s rated output.
A 15W wireless pad plugged into a weaker or mismatched brick can still work but often throttles below full speed because wireless charging already loses some power to heat and magnetic leakage.
Using the original cable and adapter—or a certified higher-output brick—can restore “fast wireless charging” performance, while cheap uncertified accessories can hurt both speed and safety.
Multiple devices on one adapter can also split available wattage and slow each charge, so ZDNET advises dedicating one wall brick to the wireless setup; 45W GaN chargers offer extra headroom.