iPad 9th Generation Fastest Charge

Your iPad9 will optimally charge, at its fastest supported charging rate, when using a USB-C 20W Power Adapter. While you can use a Power Adapter with a higher power rating, there is no benefit in do so.

In more detail…

A compatible high-power USB Power Adapter is completely safe to use with your iPad; providing that the Power Adapter itself is healthy, it will not cause any issues whatsoever. That said, if a fault were to occur, the Power Adapter has the capacity to cause considerable damage to low-power electronics.

Apple’s current range of Power Adapter’s, including those intended for Mac computers, support USB PD (Power Delivery). USB PD is an active protocol; the source (Power Adapter) and sink/load (in this case, your iPad) “negotiate” and agree a common Power Profile that will determine the charging voltage that will be applied. 

For non-PD compliant devices - or compliant devices that fail to negotiate a mutually supported Power Profile - the Power Adapter will only supply the standard USB 5V at a reduced current. This mechanism ensures that the higher charging voltages not supported by non-PD devices are not damaged by an over-voltage condition.

You should note that while your iPad can be charged with a 30W (or other) Power Adapter, it will not charge any faster than when using the USB-C 20W Power Adapter that was likely supplied with your iPad. When charged with a USB-PD Power Adapter, the maximum iPad charging rate is limited to ~20W. When using a standard USB Power Adapter, operating at 5V, the maximum charging rate is limited to 12W (i.e., 2.4A @5V).

Where the higher-power Power Adapter does provide benefit is when being used with other USB devices via a USB hub. A higher-output Power Adapter provides a greater “power budget” - allowing other devices to receive power while maintaining the iPad’s maximum 20W charging rate. For example, a USB multiport hub may itself typically require 10W of power; connected USB devices (external storage, keyboard, mouse etc) also draw additional power from the source. If the connected load exceeds the available supply, the iPad may not charge - or may charge at a substantially slower rate. In extremis, both the iPad and Power Adapter will share the connected load - and the iPad may actually discharge its battery to make-up the shortfall in available power from a connected Power Adapter.

You should note that while Apple devices generally support USB PD, they do not support other charging standards - such as QC (Quick Charge).