Capacity is not continuous consumption
A power supply is sized to deliver power within its design limits to the computer. The number printed on it is not a command to draw that amount from the outlet every hour. CPU and GPU workload, displays, storage, fans, charging devices, idle behavior and power settings change actual demand.
Do not calculate monthly cost as 0.75 kW × 24 hours unless a wall measurement really averages 750 W around the clock. That assumption would describe a different, extreme scenario rather than normal use of a 750 W-rated unit.
Build a day from workload blocks
For an invented example, four hours of ordinary work at 120 W use 0.48 kWh. Two additional idle hours at 35 W use 0.07 kWh. Together they use 0.55 kWh and cost $0.099, about 10 cents at $0.18/kWh. Repeating that exact day 30 times would cost $2.97.
A two-hour gaming session at a hypothetical 400 W would add 0.8 kWh and $0.14. Treat each workload separately, then add the energy. Do not add PSU capacity on top of a wall reading; the wall reading already includes power-supply losses.
| Workload | Wall power | Time | Energy |
|---|---|---|---|
| Office work | 120 W | 4 hours | 0.48 kWh |
| Idle | 35 W | 2 hours | 0.07 kWh |
| Optional gaming block | 400 W | 2 hours | 0.80 kWh |
Measure the boundary you intend to price
A suitable plug-in meter can measure the tower alone or an approved power strip containing the tower and displays. Label the boundary. If the monitor is measured separately, add its kWh once; if the entire desk is already measured together, do not add components again.
Observe long enough to include representative work, idle and sleep behavior. A short benchmark run can help identify peak demand but does not describe the daily energy total.
Use power management before replacing hardware
ENERGY STAR computer criteria consider several modes, including off, sleep and idle, and require power-management features for certified products. Check that the display and system enter the intended low-power states rather than remaining active overnight.
For a purchase decision, efficiency, component needs, reliability and electrical compatibility matter alongside operating cost. A larger PSU rating alone neither proves higher daily energy use nor guarantees lower consumption. Use measured wall energy and the workload you actually run.
Choose your next step
Continue with the question that matches your next decision:
Sources & further reading
Numerical examples are illustrative unless explicitly described as measured. This guide does not claim hands-on product testing. Read the methodology and editorial disclosure.