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.
Hand-calculation path
Stay on this page when you want to see the method. Move to a calculator when you have the same inputs and only want the arithmetic handled for you.
| Step | Input to choose | What you calculate | Decision point |
|---|---|---|---|
| 1 | Watts while the appliance is running | Power draw for the device or setup. | If the wattage is unclear, find the source before estimating cost. |
| 2 | Hours of use | The session, day, or month you want to estimate. | Use the runtime that matches the habit you are comparing. |
| 3 | Watts and hours together | kWh = watts x hours / 1000. | At this point you have energy use, but not cost yet. |
| 4 | Rate per kWh | Estimated usage cost = kWh x rate. | Keep fixed fees, taxes, and bill adjustments separate from this usage estimate. |
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 per kWh, the estimated usage cost is 0.10. If you are not sure what rate to use, read how to find your electricity rate per kWh.
Example: Portable heater cost
A 1,500 W heater used for 3 hours uses 4.5 kWh because 1,500 x 3 / 1000 = 4.5. At 0.20 per kWh, the estimated cost is 0.90 for that session. If that same 3-hour heater session happens every day for 30 days, the usage cost would be about 27.00 at the same rate.
Example: Fan running overnight
A 75 W fan used for 8 hours uses 0.6 kWh because 75 x 8 / 1000 = 0.6. At 0.20 per kWh, the estimated cost is 0.12 for the night. This kind of result may look small for one night, but it becomes more useful when you repeat the estimate across many summer nights.
Example: Desktop PC during a workday
A 200 W desktop computer used for 6 hours uses 1.2 kWh because 200 x 6 / 1000 = 1.2. At 0.20 per kWh, the estimated cost is 0.24 for the day. This example shows why runtime can matter even when wattage is much lower than a heater.
When to use each calculator
- Appliance Electricity Cost Calculator for daily, monthly, and yearly cost
- Electricity Cost Per Hour Calculator for one-hour comparisons
- Watts to kWh Calculator when you need energy use before cost
- kWh to Cost Calculator when you already know the kWh amount
- Electricity Cost Calculators when you want to choose by task
What this estimate does not include
This method estimates usage cost only. Real utility bills can include fixed charges, taxes, delivery fees, tiered pricing, time-of-use rates, and seasonal changes in appliance behavior.
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?
The estimate does not include taxes, fixed fees, tiered pricing, or changes in real appliance power draw. Treat it as a practical planning number, not a utility bill total. For a fuller breakdown, read why your electric bill can be higher than a calculator estimate.