Raspberry Pi 5 power sources - 2023-10-10
Will my existing Pi USB-C power source work?
Probably.
Raspberry Pi 5 can use "Legacy" USB-C 5V@3A power supplies as used with Raspberry Pi 4 in a limited way. However, for the Pi to work properly, you must either limit USB power *OR* reduce the performance. This is a choice you can configure in Pi 5. By default, USB power is limited to about 600mA.
For full performance *AND* higher USB power consumption, a USB-C Power Delivery 5.1V@5A power supply is specified. If the USB-C PD supply properly supports 5V@5A, it will probably work. It depends on multiple factors like USB-C cable specifications, quality, supply voltage tolerances, available current, etc. Note that the combination of 5V and 5A appears to be uncommon in USB-C PD power sources, be warned! Also, there is a difference between a charger and a power supply.
To simplify this complex selection, Raspberry Pi is offering its own USB-C PD *5.1V*@5A power supply!
Nice references here: Alasdair Allan's reply, and again HERE and HERE.
Official Raspberry Pi 27W USB-C US power supply links: White, (Black available later)
What about GPIO power sources/hats?
Most existing UPS hats provide 2.4A or 3A only. Using them with Pi 5 would require limiting the Pi's performance to ensure stability. As with pre-Pi 5 power sources, they should be considered "Legacy sources", thus limiting the Pi 5's performance in some ways. Manual configuration of the limits is desired in config.txt.
If you want to power your Pi 5 using the +5V and Ground GPIO pins (directly or with a HAT), you should test how much current your specific configuration requires in its most power-hungry state. If your power source meets the specifications (stable 5V±5%@5A), you could disable the performance restrictions (add a line to config.txt).
See Gordon Hollingworth's comment.