Watts become kilowatt-hours over time
Daily kWh = (active watts × active hours + idle watts × remaining hours) ÷ 1,000. Multiply by the number of days and your rate per kWh for the cost. The annual comparison always uses 365 days, independent of the selected period.
A machine using 100 W for 8 hours and 30 W for 16 hours consumes 1.28 kWh per day. At 0.20 per kWh, that is 7.68 over 30 days and 93.44 over 365 days, in your selected currency.
Should I enter the power supply wattage?
No. A 750 W power supply rating is its capacity, not your computer’s continuous draw. Measure at the wall with a suitable energy meter. Wall measurements already include power supply losses, so do not add them again.
What if the system sleeps or turns off?
For a two-state schedule, enter sleep or off-state draw in the idle field and the appropriate active duration. For three or more states, calculate their watt-hours separately or use a measured daily kWh total. This tool assumes the same schedule every day.
Before replacing hardware to save power
Measure idle draw first. Compare the annual saving with the replacement cost and expected service life. An efficient upgrade can still take years to pay back. The result here models electricity only and does not recommend a purchase.
Reference: U.S. EIA: measuring electricity.