Compare · Two watts apart
18W vs 20W: about two minutes of difference.
Two watts, one rail, the same protocol. On an iPhone the 20W adapter reaches 50% roughly two minutes sooner than an 18W one. This is the rare comparison where the honest answer is that it barely matters — and where the money is better spent elsewhere.
Spec sheet, side by side
| 18W USB-C | 20W USB-C | |
|---|---|---|
| Top rail | 9V/2A | 9V/2.22A |
| Low rail | 5V/3A | 5V/3A |
| Protocol | USB-C Power Delivery | USB-C Power Delivery |
| Ports | 1× USB-C typical | 1× USB-C typical |
| Size class | Small cube | Identical footprint |
| Best-suited devices | iPhone, Pixel, Quest 3 (18W), earbuds | iPhone, iPad, Kindle, earbuds |
Both sit on the same 9V rail and both speak the same standard, so the phone negotiates an almost identical contract either way. The 11% current difference is inside the noise created by battery temperature and how many background apps are syncing. On an iPad, where the device would happily take 30W, the two are equally unsatisfying.
Where each one wins
18W wins on nothing you can measure — except it is already in your drawer. Apple shipped an 18W USB-C brick with the iPhone 11 Pro and 2018 iPad Pro, and it remains a perfectly good phone charger. A Meta Quest 3 draws about 18W, so an 18W unit is an exact fit there rather than an underspec.
20W wins as the current standard. It is what Apple sells today, what every third-party maker builds, and what device documentation assumes. If you are buying anything at all, 20W costs the same as 18W and has marginally better headroom. Per-class detail lives at 18W adapters and 20W adapters.
Verdict: don't replace an 18W; if buying new, skip both
Judged on delivered watts, protocol and size the two are functionally identical, so the tie-breaker has to be headroom per dollar. That points past both of them. A 30W GaN adapter occupies the same pocket, costs one bracket more, and adds a whole device class — see 20W vs 30W for the numbers. Buy 20W only when you specifically want the smallest, cheapest certified brick for a bag or a guest room; keep an 18W in service until it dies. The genuine upgrade path is 5W to 20W if you're still on a legacy cube, and iPhone fast charging explains what the phone is actually doing during that first half hour.
- 20W
Apple 20W USB-C Power Adapter
Today's standard single-port brick. Same rails as the 18W it replaced, plus 0.22A.
- Output
- 20W USB-C PD
- Rails
- 5V/3A, 9V/2.22A
- Ports
- 1× USB-C
- Value
Amazon Basics 20W USB-C
The cheapest sensible way to add a second 20W to a nightstand or a suitcase.
- Output
- 20W USB-C PD
- Ports
- 1× USB-C
- Certified
- UL listed
- Pick
Anker Nano 30W
The upgrade that actually changes something: iPhone at full speed plus iPad headroom.
- Output
- 30W USB-C PD
- Ports
- 1× USB-C
- Tech
- GaN
Apple's legacy brick has its own page at Apple 18W USB-C; the full index is at charger comparisons.
FAQ
Is an 18W charger too slow for a modern iPhone?
No. An iPhone's fast-charge window uses the 9V rail, which an 18W adapter supplies. You lose roughly two minutes to 50% compared with a 20W unit.
Why did 18W become 20W?
Apple's 18W brick used a 9V/2A profile; the 20W replacement raised that to 9V/2.22A and became the wattage third-party makers standardised on. It was a product-line change, not a new standard.
Does an 18W adapter fast-charge an iPad?
Partly. An iPad will accept 18W and charge faster than on a 12W USB-A brick, but iPad Air and iPad Pro can take around 30W, so you are leaving a third of the speed unused.