[crote]

This is not a given, though: it has guaranteed forward compatibility, not backward compatibility.

Charging a laptop from a 20W phone charger usually doesn't work because the laptop will have a minimum voltage/current requirement, but charging a phone from a 240W laptop charger will always work.

[kiwijamo]

That has been my experience as well. I've not come across a laptop that accepts a charge from a low power USB-C PD charger. I'm aware the standards allow for falling back to lower voltages but it seems laptop manufacturers specifically go out of their way to stop this. I presume to avoid issues with their battery charging system.