How it is calculated
Energy in use = (computer + monitors × count) × hours × days ÷ 1,000.
Standby energy = standby power × remaining hours in the month (average 30.4-day month) ÷ 1,000. Cost is the sum times the price per kWh.
Worked example
Using the calculator’s example values:
- Average computer power in use: 120 W
- Power per monitor: 25 W
- Number of monitors: 1
- Hours of use per day: 8 h/day
- Days of use per month: 22 days/month
- Left in standby the rest of the day: Yes
- Standby power (whole setup): 3 W
- Electricity price: $0.15/kWh
Result: Estimated monthly cost — $4.08 (annual cost: $48.93; monthly consumption: 27.18 kWh; cost per hour of use: $0.02).
Money-saving tips
- Enable automatic sleep after a few idle minutes.
- Lower monitor brightness: it is one of the biggest factors in its consumption.
- A switched power strip cuts standby draw from peripherals.
Frequently asked questions
Is the PSU rating what it draws?
No. The PSU rating is a maximum; real draw is usually much lower except under heavy load. A plug-in meter gives the exact figure.
Does a laptop use much?
It usually draws much less than a desktop. Use the charger’s wattage as an upper limit.
Shut down or sleep?
Sleep uses very little but not zero. If you won’t use it for many hours, shutting down or switching off the strip removes it.
How we calculate and the limits of the results: Methodology.