A Watt is a unit of electrical power, which expresses how much
energy (measured in Joules) gets used or transmitted per second. A Volt
is a measure of electric potential. It expresses how much energy you
get per unit charge, with no mention of time.
1 Watt = 1 Joule/Second
1 Volt = 1 Joule/Coulomb
(a Coulomb is a unit of electrical charge).
If you happen to know the current flowing through something, measured in Amperes (affectionately called "Amps"):
1 Ampere = 1 Coulomb/Second
you can figure out how many Watts of energy are used if you know also the voltage:
1 Watt = 1 Ampere * 1 Volt
So you need to know two things in order to figure out the third.
It's a lot like asking how to convert miles into hours. You need to know speed too!
(republished on 08/02/06)