Volts are the unit of the “pressure” (voltage) of electricity. Even if you don’t switch on a device, the battery provides it’s voltage. You need Volts first in order to do anything with electricity.
Applying Volts to a circuit then causes an electrical current to flow, it’s measured in Amperes.
Watts are the unit of power. Your phone charges with 5 Volts from USB, probably causing 2 Ampere to flow.
Volt x Ampere = Watt
5V x 2A = 10W
Your phone therefore charges with a power of 10 Watts.
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