Why do we speak of watts (and not amps) when we refer, for example, to the "7.5W Wireless Charger for iPhones," but when we talk about cord-based USB charging, the specs typically refer to amps (and not watts). For example, one page explains that the "USB spec is 500 mA maximum current. The charger provided with your iPhone can provide up to 1000 mA."
The difference between amps, watts, and volts is something I have struggled to understand since high school, which was a long time ago for me. I've re-read your standard references like Wikipedia, but I'm still not getting it.
Can someone help me understand these concepts by demonstrating how I would apply them to understand how to compare the difference, in performance, between a 7.5W wireless iPhone charger and a 1000 mA corded USB charger?
Thanks in advance for your help and patience.