Start only when the input source is clear
This page is for a chosen watts-and-runtime scenario. It does not decide whether a label wattage is representative of a cycling appliance, and it does not replace a kWh reading you already measured. For that decision, use the watts vs kWh guide or price a labeled period with the kWh to Cost Calculator.
The basic formula
The basic formula is cost = watts x hours / 1000 x electricity rate per kWh. Dividing by 1000 converts watts into kilowatts, and multiplying by hours gives the energy use in kWh.
This guide focuses on the calculation path from watts and hours to kWh and then to cost. If you only need the difference between watts and kWh, start with the watts vs kWh guide.
If you want the calculator to handle the formula, use the Appliance Electricity Cost Calculator for daily, monthly, and yearly estimates.
Check each formula step before moving on
Use this table to pause at the step where an input is missing. Move to a calculator only after watts, runtime, and rate are clear enough for the estimate you want to make.
Scroll horizontally to see all columns.
| Formula step | Input needed | Stop if... | Continue when... |
|---|---|---|---|
| Watts to kW | Appliance watts | You only have volts, amps, or a mode label. Find wattage first, or use known kWh when you measured energy use. | You have a watt value for the device or setup you are estimating. |
| Runtime to kWh | Hours of use | The runtime does not match the session, day, or month you want to estimate. | You have hours that match the habit or period you are comparing. |
| kWh to cost | Rate per kWh | The number is a total bill amount, fixed charge, or unclear bill line. | You have a per-kWh rate or a rough fallback that you will label as approximate. |
| Interpret result | Usage-cost estimate | You are comparing it directly with the full bill total. | Bill-level items such as fixed fees, taxes, delivery charges, and adjustments stay separate. |
Input
Watts
Power draw while the appliance is running.
Energy
kWh
Watts multiplied by hours, divided by 1000.
Estimate
Cost
kWh multiplied by your electricity rate per kWh.
Step 1: Find the appliance wattage
Look for watts on the appliance label, manual, product page, or specification sheet. If you are not sure where to find the number, start with the How to Find Appliance Wattage guide.
Step 2: Estimate hours of use
Runtime matters as much as wattage. A 1,500 W heater used for 1 hour has a very different cost from the same heater used for 8 hours. Use a realistic average for everyday estimates instead of the longest possible runtime.
Step 3: Convert watts and hours into kWh
Multiply watts by hours, then divide by 1000. For example, 100 W used for 5 hours is 0.5 kWh. If you only need this conversion step, use the Watts to kWh Calculator.
Step 4: Multiply by your electricity rate
After you have kWh, multiply by your rate per kWh. If the appliance uses 0.5 kWh and your rate is 0.20 currency units per kWh, the estimated usage cost is 0.10 currency units. If you are not sure what rate to use, read how to find your electricity rate per kWh.
Worked calculation: 100 W for 5 hours
First convert the energy use: 100 W x 5 hours / 1000 = 0.5 kWh. Then apply the sample rate: 0.5 kWh x 0.20 currency units per kWh = 0.10 currency units. That is a complete formula walkthrough for one selected period.
For an appliance-specific decision, use the relevant scenario after the formula is clear: a heater session, overnight fan use, or a home office setup each needs its own runtime or measurement boundary.
Keep the selected period with the result
State whether the runtime is one session, one day, or a repeated pattern before scaling. If the same pattern is not likely to repeat, keep the result as that selected period instead of turning it into a monthly estimate.
Keep the rate source with the estimate
This method prices selected usage with the rate you enter. A listed energy-only rate and a blended bill-average rate can reflect different charge components, and neither turns one appliance calculation into a household statement.
FAQ
Can I calculate cost from watts without knowing hours?
No. Watts describe power while the appliance is running, but cost depends on how long it runs. If the difference is unclear, read Watts vs kWh Explained.
Should I use daily hours or one session?
Use daily hours when estimating a regular habit. Use one session when comparing a single evening, workday, or short appliance use.
Why might my real bill differ from the estimate?
First check whether the selected runtime, wattage, and rate source match the period you are comparing. The result still describes the selected appliance use rather than the whole household statement. For a fuller breakdown, read why your electric bill can be higher than a calculator estimate.