The amount of watts that an led computer monitor uses can vary from monitor to monitor depending on the size of the monitor as well.
My 21.5" led computer monitor uses 30 watts when hooked up to a Kilowatt meter.
If that monitor were a crt monitor it would use as much as 80 to 100 watts of electricity.
Led monitors use very little electricity and should only cost you around $1.00 to $2.00 per month or even sometimes less to operate depending on how much you use your led monitor.