If you are connecting high power USB devices to your hub, you might be best advised to connect your Power Adapter to the USB PD (Power Delivery) port of the Hub. The hub is best considered as optimum distribution point for both power and data.
If instead you attempt to power the iPad and connected peripheral devices from the Magic Keyboard’s spine connector, you are unlikely to achieve optimal power needs of connected devices. When using the spine connector, all power must be passed from the Keyboard via the Smart Connector to the iPad - and then via the USB-C port to the Hub and connected peripherals. At best, even if the hub connection can draw sufficient power from the iPad, the iPad’s electronics will be under greater load, adding to heat generation and thermal management. At worst, insufficient power will be available to peripherals.
Consider your power-budget…
- For your iPad to charge at its maximum supported charging rate, it will require 20W. Actual power needs will fluctuate according to state of charge, screen brightness and CPU load; the instantaneous load is shared between the iPad battery and the connected Power Adapter. Whilst fundamentally limited by available power, the iPad’s thermal management will control the achievable charging rate.
- The USB hub also requires power. Hub power requirements will vary by manufacturer and sophistication; some manufacturers quote the power requirements, but in absence of a definitive figure, perhaps allow 10W for the Hub itself. Ethernet LAN, HDMI, VGA, SD/microSD Card interfaces are included within the Hub power requirements.
- USB Type-A ports of your hub supply power to run the connected USB peripheral device. USB Type-A are typically designed to provide 0.5-1.A @5V (i.e., 2.5-5W), although some may be designed to downstream charge at up 2.4A (i.e., 12W). Unless internally powered, such as camera with internal batteries, USB devices will always require some power - but many (such as keyboard and mouse) can be estimated at the lower end (~1.0W), whilst connected USB storage are considerably more power hungry.
- Any USB-C peripheral ports will also provide power to connected USB-C devices. Again, the hub specifications may specify the individual power capabilities of the ports - otherwise, estimate as you would for connected USB Type-A devices.
Adding-up the combined total for the hub (~10W), connected devices (?) and the iPad at its maximum demand (~20W) will provide the ideal minimum power rating for your Power Adapter - higher is good as the Power Adapter itself can run more efficiently while generating less heat.
Your USB Hub will have a quoted USB-PD pass-through power rating; for your iPad to charge at its maximum charging rate, this needs to be rated at 20W or greater. When using a USB-C Hub with an iPad, I would recommend using a USB PD Power Adapter rated at 30W or more.
