Calculate monitor running cost.
Monitor running cost is watts times hours divided by a thousand to reach kilowatt-hours, multiplied by your tariff. Standby is treated separately because it applies for the other fifteen or so hours of every day and, at well under a watt on modern panels, turns out to be negligible — which is a more useful conclusion than an unspecified worry about leaving screens on.
Daily energy
Daily kWh = (watts x active hours + standby watts x idle hours) x monitors / 1000
Annual cost
Annual cost = daily kWh x 365 x rate per kWh
Considerably on LED-backlit panels — dropping from full to fifty percent brightness commonly cuts draw by a quarter to a third, since the backlight dominates consumption.
The arithmetic says it barely matters. At 0.4 W standby, a year of overnight idling costs a few pennies. Turning them off during the day when unused saves far more.