Watts vs. kilowatt-hours: calculate what a device costs to run
Quick answer
To estimate a device’s electricity cost, divide its average power in watts by 1,000, multiply by hours of use, then multiply by your price per kilowatt-hour. A device averaging 50 watts for four hours uses 0.2 kilowatt-hours. At an assumed $0.20 per kilowatt-hour, that costs four cents.
The arithmetic is simple. Choosing a realistic wattage, time period and electricity rate takes more care. These examples use invented household scenarios, not measurements of particular products.
What the two units tell you
Watts, abbreviated W, describe power at a moment. Kilowatt-hours, abbreviated kWh, describe energy used over time. One kilowatt is 1,000 watts. A steady 1,000-watt load running for one hour consumes 1 kWh; a steady 100-watt load needs ten hours to use the same amount. EIA’s explanation of electricity measurements covers this distinction.
Think of watts as the number you check when a device is doing something now. Think of kWh as the total you accumulate during a day, week or billing period. You pay for the accumulated energy under your utility’s rate structure.
Use this formula:
Energy in kWh = average watts × hours ÷ 1,000
Estimated energy cost = kWh × dollars per kWh
If your bill quotes cents, convert them first: 20 cents is $0.20, not $20.
Example 1: a desk setup
Suppose a monitor and its accessories together average 50 W while in use. You use them for four hours per workday, on 22 days this month.
Time: 4 × 22 = 88 hours
Energy: 50 × 88 ÷ 1,000 = 4.4 kWh
Cost at the assumed $0.20 rate: 4.4 × $0.20 = $0.88
That estimate covers the working hours only. If the equipment stays plugged in outside those hours, measure or estimate its other operating modes separately. A device that appears off can still draw standby power, as the Department of Energy explains.
For this hypothetical setup, assume a measured average of 2 W during the remaining 632 hours of a 30-day month. That adds 1.264 kWh, or about $0.25. The combined estimate becomes approximately $1.13.
Example 2: a small always-on device
Now suppose a network device averages 10 W continuously:
Daily energy: 10 × 24 ÷ 1,000 = 0.24 kWh
Thirty-day energy: 0.24 × 30 = 7.2 kWh
Thirty-day cost at $0.20 per kWh: $1.44
In this example, the lower-wattage device uses more energy than the desk setup because it runs longer. That comparison can help you decide what to measure next. It does not establish that either device is wasteful or should be switched off. Network equipment may support cameras, calls and other services your household relies on.
Example 3: use a measured energy total
For a device that changes power frequently, start with a cumulative kWh reading rather than one momentary watt reading. Suppose a suitable meter records 2.1 kWh during seven representative days.
Daily average: 2.1 ÷ 7 = 0.3 kWh
Thirty-day projection: 0.3 × 30 = 9 kWh
Projected cost at $0.20 per kWh: $1.80
This is a projection from that week. A holiday, a heat wave or different settings can change the next month. DOE’s measurement guidance explains why measuring energy over time is useful when power fluctuates.
Find the rate that matches your question
For an estimate of a device’s additional usage cost, look for the charges on your bill that vary with kWh. Supply and delivery may be listed separately. Fixed monthly customer charges usually do not disappear when you use less electricity, so allocating them to a device can overstate the amount you could avoid.
Dividing the entire bill by total kWh can give you a rough all-in household average. Label it that way rather than treating it as a guaranteed savings rate.
If your plan has time-of-use prices, calculate the energy used in each price period separately, then add the costs. EIA notes that some utilities offer time-of-day pricing. Check your own tariff for seasonal prices, tiers, taxes and other charges. When the bill is unclear, ask the utility which rate applies to an extra kWh at the time you use it.
Choose a useful wattage
A maximum rating on a power supply is not a measurement of typical consumption. A “100 W” adapter does not prove that its connected device draws 100 W throughout the day. Prefer a credible average measurement or a model-specific energy estimate with stated conditions.
For major appliances, an EnergyGuide label can help with comparisons. Its annual cost is an estimate based on typical use and an average energy price; your household may differ. See the FTC’s EnergyGuide explanation. Do not improvise a plug-in measurement on hardwired or high-load equipment.
Your five-line worksheet
Write down:
Device and mode: working, idle, charging or standby
Average watts, or directly measured kWh, and where the number came from
Hours or measurement dates, including how typical they were
Electricity rate and whether fixed charges are excluded
Calculated cost, rounded sensibly, plus the biggest uncertainty
Keep the original readings. If the estimate seems surprisingly high, check the decimal point, cents-to-dollars conversion and hours before buying replacement equipment. Good measurements can improve a decision, but this calculation alone cannot promise savings or justify a new purchase.
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