The electricity used by a second monitor depends on the particular display, its settings and the hours it operates. A wattage printed near the power input often describes an electrical rating rather than typical everyday use. Estimate running cost from an appropriate measured or documented consumption figure and distinguish active operation from standby before comparing alternatives.
Define what the estimate includes
Decide whether the question concerns the monitor alone or the whole desk. A display that also charges a laptop or powers USB devices can draw electricity for several purposes. Comparing that reading with a screen that supplies only video can be misleading. State the measurement boundary and account for the connected equipment instead of attributing every watt to the panel.
Look for manufacturer consumption information with its stated test conditions. Screen size, brightness, HDR content and enabled features can change active demand. Two monitors with similar dimensions can therefore have different real consumption, and the same monitor can vary during a workday. Treat a specification as a conditional reference rather than a precise personal annual total.
For a practical observation, use a suitable electricity meter according to its instructions and the equipment's electrical requirements. Measure over representative work, including ordinary brightness and any charging load you intend to include. A single instantaneous reading during a bright scene or a charging surge may not represent the average use that determines the bill.
Convert power and time into a useful comparison
Energy in kilowatt-hours is power in kilowatts multiplied by operating hours. A hypothetical 25-watt display used for eight hours consumes 0.2 kilowatt-hours during that period, excluding any additional standby time. Multiply the estimated energy by the applicable electricity rate, keeping the example separate from a claim about a particular product's actual measured demand.
Consider the hours when the second screen has no useful task. The operating system's display-off timer or the monitor's supported standby behavior may reduce unnecessary active time. Check that the chosen settings resume reliably and do not interrupt the work. A screen saver with moving content should not be assumed to save the same energy as turning the display off.
Reduce brightness only to a level that remains comfortable and readable in the room. Working with a dim unreadable screen to achieve a small consumption change can undermine the purpose of the workstation. Room lighting and glare control may help establish a sensible setting, while model-specific power features should be assessed for their effect on the task and wake behavior.
Compare alternatives using the same assumed hours, measurement boundary and electricity rate. Include uncertainty where usage varies, and revisit the estimate after an actual schedule change. This produces a useful planning range without pretending that a generic wattage predicts every household's bill or that adding a second monitor has the same energy consequence in every workspace.