A very higher proportion of computer USB ports are limited in their ability to source power, typically restricted to 0.5A @5V (2.5W) or 1A @5V (5W). Most models of iPad require a minimum of 10-12W.
External USB Power Adapters come in a variety of specifications - they are not equal in their ability to universally power, or charge, connected devices.
An Apple Power Adapter of 10/12W minimum will charge an iPad. Power Adapters supplied with many models of iPhone are designed to provide 5W. The maximum charging rate for USB-PD capable iPads are 18W (12V @1.5A); others are limited to 12W (5V @2.4A).
Apple’s Power Adapters rated 18W or greater, including those supplied with a Mac Computer, are USB-PD compliant. This indicates that they conform to the USB-PD standard; in addition to 5V, these Power Adapters can negotiate a higher charging voltage and current - and can Fast Charge compatible iPads and iPhones.
Do not confuse the USB-PD (Power Delivery) standard with QC (Quick Charge) or other proprietary fast charging standards (e.g. Huawei SuperCharge). USB-PD, QC and proprietary standards are not interchangeable. Whilst USB-PD and QC Power Adapters are designed to fall-back to 5V at a low charging current when connected to incompatible devices, other Power Adapters may not do so - and may cause damage, voiding any device warranties.
USB-PD compliant devices complete an initial data-handshake when first connected, between the device and the Power Adapter, during which the charging voltage and current is mutually agreed. If the conversation fails, the Power Adapter should only supply the basic USB 5V - current limited to a “safe” value (typically 2.0-2.4A).
Given the high cost of an iPad and other Apple devices - and the relative low cost of an official Apple Power Adapter - is it worth the risk to use a Power Adapter that is not specifically designed for Apple products?
I hope this information and clarification prove to be helpful.
