1 watt = 1 joule/sec
(1800 joule/sec) x (20 minutes) x (60 sec/minute) = 2,160,000 joules
(2.16 megajoules)
I wouldn't exactly say that this energy was "used" by the iron.
What the iron did was to take 2.16 megajoules of electrical energy
and convert it into 2.16 megajoules of heat . . . something that the
things we build are very good at, and which we know how to do with
nearly 100% efficiency.
