Here are the basics. It is pretty linear. 240 volts being twice as much voltage will draw half the amount of current. The use of current is what they charge you for.
EX 120 volt drawing 20 amps would only draw 10 amps soit uses half the electricity thus less expensive to run..
Good enough?
WB
This is absolutely not true....
Edux10 here is the correct answer.
240volt supplies a device at a higher current allowing it to draw less AMPS.
ie. a 1000 watt light on 120 volt will draw 9.8 amps, where as on 240 volt it will draw 4.9 amps.
It is still using 1000 watts per hour... the electricity bill is the exact same either way.
240 volt is standard in europe however 120 volt is standard in the US.
all 240v does is allow you to power more devices off of the same breaker.