I see that the official one will do 5A at 5(.1)V which I make 25.5W but is said to be 27W.
What do USB C laptop chargers typically supply at 5V? The one in front of me says it will supply 100W, but that’s because with PD it will do 20V at 5A. It says the 5A supply is 3A but is that just because that’s the spec cables have to follow, or is it likely to be OK to supply more at 5V?
It would seem odd that the Pi PSU will do 5A and a higher rated one wouldn’t, but the whole field is covered in oddities.
If it says 5V 3A then that’s what it will deliver and no more. USB C laptop chargers use USB Power Delivery to negotiate this, and 5V 3A is a standard combination which all (?) PD supplies will do. 5V 5A is allowed by the specification, but is unusual and needs a few other things to be USB PD compliant. Unfortunately very few chargers seem to do that and so won’t provide 5V 5A, so if you want to use USB devices with your Pi 5 you’re best sticking to the official charger.
One other thing to take into account: chargers are not optimized for stable voltages. I have tested the Pi5 with some other power adapters. They do work, and the Pi5 in normal use does not draw that much current after all, so an 5V/3A USB-C power-adapter is ok (especially if you use the Pi4-power-adapter). Nevertheless, with no-name adapters I sometimes had the “yellow flash” signaling low voltage.
As @Shoe pointed out, if you attach two large USB-HDD (or other USB-devices) than you might run into trouble (USB3 has to deliver 0.9A per port, but the Pi5 limits the USB-power to 1.6A in total, which is still a lot).
Keep in mind that if you use a 5V/3A USB-C power supply, the Pi 5 will limit the max USB current to 600ma. Introducing: Raspberry Pi 5! - Raspberry Pi
When using a standard 5V, 3A (15W) USB-C power adapter with Raspberry Pi 5, by default we must limit downstream USB current to 600mA to ensure that we have sufficient margin to support these workloads. This is lower than the 1.2A limit on Raspberry Pi 4, though generally still sufficient to drive mice, keyboards, and other low‑power peripherals.
Powering the Pi 4 from the official PSU along with a “few” LEDs attached to the power rail, I quickly reached a point where I was asking too much. In the end I swapped over to a 5V 7A Mean Well PSU. Wasn’t too hard to wire a USB C connector to it.
It’s mentioned in a couple of threads there, but more as a side note. I did a quick search but couldn’t find it. I use the same username alphanumeric, there.
The official documentation seems to suggest that this is for the Pi 5 only though, so I don’t know if it would have mattered if @major_tomm was using a 4.
Power supply properties /chosen/power
Raspberry Pi 5 only.
usb_max_current_enable - 32-bit integer
Zero if the USB port current limiter was set to the low-limit during boot or non-zero if the high limit was enabled. The high level is automatically enabled if the power supply claims 5A max-current OR usb_max_current_enable=1 is forced in config.txt