Monthly usage (kWh) = power (W) Γ hours per day Γ days per month Γ· 1,000. Multiply by the rate per kWh to get the monthly cost, and by 12 for the yearly cost. For example, a 300W PC used 8 hours a day for 30 days at 120 per kWh uses 72 kWh and costs about 8,640 per month.
For desktops, base the calculation on actual usage rather than the power supply (PSU) rated output. A typical office PC draws 100-200W, a gaming PC 300-600W, with monitors adding another 20-50W. For an accurate figure, a plug-in wattmeter shows real-time consumption.
Set auto shutdown instead of a screensaver, and use a power strip that cuts standby power when the PC is idle. A high-efficiency 80 PLUS certified power supply also reduces energy loss for the same workload.