Distinguish screen-off from system sleep
ENERGY STAR’s computer guidance emphasizes power management and explains that screen savers do not provide the same savings as low-power states. A display can turn off while the computer continues downloads, processing or a running game.
Check the operating-system settings and the behavior you observe after the chosen timeout. If a work application prevents sleep, account for that actual state rather than calculating the advertised sleep figure for hours the machine never spends asleep.
Put idle, sleep and shutdown on the same schedule
These are illustrative averages, not benchmarks, typical values or tests of a particular computer. All rows cover 12 hours each night for 365 nights at $0.18/kWh. Daytime work and peripherals are excluded unless included in the assumed watts.
| State | Assumed average input | Energy per year | Cost per year |
|---|---|---|---|
| Active idle | 50 W | 219 kWh | $39.42 |
| Sleep | 3 W | 13.14 kWh | $2.37 |
| Shutdown, plugged in | 0.5 W | 2.19 kWh | $0.39 |
Using unrounded values, sleep instead of active idle avoids 205.86 kWh, worth $37.05; shutdown instead of sleep avoids 10.95 kWh, worth $1.97. Those are different decisions. A computer that fails to sleep can be a larger opportunity than an already low shutdown draw.
Edit the overnight sleep example →
The calculator uses 365 ÷ 12 days per month to make a 365-day year. Enter your measured average for the period and change the hours to those actually unattended.
Measure the state you intend to change
Use a suitable meter over a long enough period to resolve small loads, with a device connection allowed by the equipment instructions. Record whether peripherals and a powered dock are included. Check that scheduled activity did not wake the machine during the test.
An overnight measurement is a description of that particular night. Repeat if updates or other unusual tasks made it unrepresentative.
Keep work requirements in view
Save work and allow a normal shutdown when that routine suits your needs. Sleep may offer convenient resumption; scheduled tasks may require another arrangement. For an employer-managed computer, follow the organization’s settings and update requirements. Compare the modest energy difference with the routine you can reliably maintain, and prioritize preventing long periods of unintended active idle before buying hardware solely to reduce a few watts of standby.
A practical choice for your routine
| Situation | What to check first |
|---|---|
| You resume work frequently | Does sleep work reliably and preserve the workflow you need? |
| You finish for the day | Can you save work and shut down normally without interrupting required tasks? |
| The screen is dark but the system stays active | Check power settings, background work and scheduled wake-ups. |
| The device is employer-managed | Follow the organization’s update and access requirements. |
For a laptop, charging the battery can affect a short wall-power reading; use a representative observation instead of comparing one charging session with one already-full battery. For a desktop, record whether the display and dock are included. Read how to measure appliance electricity before assigning a saving to your own computer.
Choose your next step
Continue with the question that matches your next decision:
- Gaming PC Electricity Cost: Use Wall Power, Not PSU Size
- Laptop Charging Cost: Per Charge, Month and Workday
- Printer Standby Electricity: Measure the Week, Not One Page
- Does a 750W Power Supply Use 750 Watts?
- Mini PC Electricity Cost per Month Running 24/7
- Dual Monitor Setup Electricity Cost per Month
- Laptop vs Desktop Electricity Cost per Month
Shopping options for this guide
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P3 P4400 Kill A Watt electricity usage meter
The P4400 displays watts and accumulates kWh. Use the energy reading with your electricity rate to estimate the cost of compatible plug-in equipment.
Before buying: For compatible North American outlets: 115 V operating voltage, 125 V maximum and 15 A maximum. Not for 230 V outlets or hardwired appliances. Check low-power accuracy before relying on tiny standby readings; record totals before unplugging, which resets the counters.
View P3 P4400 Kill A Watt electricity usage meter at Amazon.com (opens in a new tab)These are shopping options, not hands-on test results. Check the retailer for current specifications, price and availability.
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.