On Sat, 04 Nov 2017 17:05:34 +0100, Tim Streater wrote:

AFAIK, such sockets are rated at 5V, 1A max - that's the spec.

Orginal USB spec is 5 V +/- 0.25 V and 500 mA max.

This should mean that my iPhone when charging won't draw more than 1A,
but I don't know whether devices are clever enough to "try it on", try
sucking out more than 1A and look for voltage sag, give up if so.

Depends on the source, is it just a 5 V 1 A "power supply" or is it a
5 V 1 A "USB charger" or a 5 V 1 A "USB port" on a laptop/desktop.

My phone refuses to charge from a "power supply" but is quite happy
from a "USB charger" or "USB port". The latter two will have the
required resistors that tell the device what charge rate(s) are
available. A "USB port" will have the resistors but also monitor the
current draw and shut the port down if too much is taken. There may
even be a data conversation to agree power requirements rather than
just rely on the resistors.

--
Cheers
Dave.