What does your computer use in a day?
Turn watts and hours into an electricity estimate. Separate active work, idle time and sleep, then explore what changes when some idle time becomes sleep.
Start with an example, not a device claim. The starting values are illustrative: 100 W for 4 active hours, 40 W for 4 idle hours and 2 W for the remaining 16 hours. They are not measurements or guarantees for any computer. Replace them with your own readings.
A day, a month, a year.
- Per day
- kWh
- 30-day month
- kWh
- 365-day year
- kWh
| State | Watts | Hours | kWh/day |
|---|---|---|---|
| Active | |||
| Idle | |||
| Sleep |
Display values are rounded; calculations use unrounded numbers. A nonzero value below the displayed precision is shown with a < sign.
Watts are power. kWh is energy.
Watts describe how quickly a device uses electrical energy. Multiply watts by the time spent in each state, add the results, then divide by 1,000 to get kilowatt-hours. The U.S. Energy Information Administration explains these units.
Sleep hours = 24 − active hours − idle hours
Daily kWh = [(active W × active h) + (idle W × idle h) + (sleep W × sleep h)] ÷ 1,000
Period kWh = daily kWh × days
Estimated energy cost = period kWh × your price per kWh
Measure the right thing
Use a suitable plug-in power meter, within its ratings and instructions, to measure the whole device at the wall over representative workloads. Record active, idle and sleep separately. A processor’s TDP or an adapter’s rated output is not a measurement of whole-device wall power. A monitor, dock, speakers or other devices are not included unless their draw is included in your inputs.
The U.S. Department of Energy’s Energy Efficiency in the Home educational guide (PDF) uses watt-meter exercises to show why consumption varies with use and why equipment labels do not tell the whole story. Its old example appliance figures and prices are not used here.
A simple model, not your electricity bill
The calculation repeats the same 24-hour pattern every day. A month uses your selected 1–31 days; a year always uses 365 days, not twelve of your selected months. It assumes a constant average wattage in each state. It does not model charging separately, seasonal changes or different weekday and weekend patterns.
Cost appears only when you supply a rate. The single-rate estimate does not add fixed charges, demand charges, tiered pricing, time-of-use changes or taxes not included in the rate you enter. It is an educational estimate, not financial advice or a savings guarantee.
Save energy without weakening security
Check device and workplace policy before changing sleep behavior. Keep screen locking, authentication, security updates and required monitoring in place. Consider backups, remote access and scheduled jobs; moving hours to sleep is not suitable for every device or task.