How to measure a device’s electricity use
For a suitable plug-in device, a meter that records cumulative kilowatt-hours lets you turn real usage into an electricity-cost estimate.
Watts describe power at a moment. Kilowatt-hours record energy used over a period, including changes between busy and quiet hours.
From a meter reading to a cost estimate
Check that the meter fits the job
Match the meter’s plug, voltage, current limit, and permitted load to your device. Follow the manufacturer’s setup instructions. This guide covers compatible plug-in equipment; it does not cover hardwired circuits or opening an electrical panel.
Record your starting point
Note the initial cumulative kWh and the start time, or reset the energy counter if the instructions allow it. Keep a record of which device or group of devices is included.
Run a representative schedule
Include normal work, idle time, and standby time. A full day can describe a daily routine; use several days when usage varies. For a server with weekly backups, include a complete backup cycle.
Read the energy used and elapsed hours
Subtract the starting kWh reading from the ending reading. Use the actual elapsed hours. Note unusual events so you do not mistake an exceptional period for everyday use.
Calculate average watts
Multiply the measured kWh by 1,000 and divide by elapsed hours. Use that average in the calculator with the same schedule you measured.
Worked example: 7.2 kWh in 72 hours
Average watts = 7.2 × 1,000 ÷ 72 = 100 W
At 18¢/kWh, the measured period costs $1.30 after rounding. If that same pattern continues all year, the estimate is $13.15 per average month and $157.68 per year.
In the running-cost calculator, enter 100 as both active and idle watts, set total hours on to 24, and enter your actual rate. The active-hour split then makes no difference.
If your measurement already includes overnight shutdown or standby time, keep the 24-hour schedule when using its whole-day average. Reducing the hours again would count the off-time reduction twice.
Use your measured wattsWhat to look for in an electricity meter
Needed for this method
- Cumulative energy in kWh.
- Compatible voltage, plug, and load rating.
- An accuracy specification covering the power range you want to measure.
Helpful to check
- An elapsed-time counter.
- Whether readings survive unplugging or a power cut.
- A display you can read where the device is connected.
A meter that shows only instantaneous watts cannot directly give you the accumulated kWh used in this method. Small standby loads also need a meter with suitable low-power resolution and accuracy.
As a manufacturer example, the P3 Kill A Watt P4400 lists cumulative kWh measurement, 115 VAC operation, a 125 VAC maximum, and a 15 A maximum. Check the current specifications and instructions against your own equipment before choosing a meter.
The P3 link is a specification reference. We have not tested this product, and the link is not an affiliate link.
Three details that change the answer
Standby still belongs in the measurement
A device can use electricity while appearing to be off. Include that period if you want the cost of leaving it plugged in.
Variable rates need more than one price
If electricity prices change by time of day, split the measured energy by price period where possible. Multiply each period’s kWh by its rate and add the costs. A single unweighted average can misstate the result.
Measure the same equipment before and after
For an upgrade comparison, use comparable workloads and measurement boundaries. A whole equipment group’s reading should not be compared with one component’s reading as if they covered the same job.
Sources and calculation notes
The worked example uses an illustrative 18¢/kWh rate, 30.44 days per month, and 365 days per year. It is a calculation, not a product test or a prediction of your bill.