This electricity cost calculator estimates device running cost from watts, hours per day, days, and price per kWh. Defaults are 100 watts, 5 hours, 30 days, and rate 0.15. Energy use is ((100 × 5 × 30) / 1000) = 15 kWh. Cost is 15 × 0.15 = 2.25.
Use it for single appliance comparisons and classroom bill sketches. A full utility statement can include fixed charges and tiered rates that this page does not model. For percentage comparisons of two costs after you have totals, the percentage calculator can help with relative change math.
How the formula works
Cost = ((watts × hours × days) / 1000) × rate per kWh. Watts times hours times days produces watt hours for the period. Dividing by 1000 converts to kilowatt hours. Multiplying by the rate prices that energy.
Worked example
Set watts to 100, hours to 5, days to 30, and rate per kWh to 0.15. Compute 100 × 5 × 30 = 15000 watt hours. Divide by 1000 to get 15 kWh. Multiply by 0.15 to get 2.25. That is the default cost.
| Input | Value |
|---|---|
| Watts | 100 |
| Hours per day | 5 |
| Days | 30 |
| Rate per kWh | 0.15 |
| Energy | 15 kWh |
| Cost | 2.25 |
How to use the fields
- Watts is the device power draw used for the estimate.
- Hours is daily operating time.
- Days is how many days that daily pattern repeats.
- Rate per kWh is the price you apply to the computed kilowatt hours.
Nameplate watts versus real draw
Nameplate ratings can exceed typical running watts. If you have a measured draw from a meter, prefer that value. Using a peak rating for a device that rarely hits peak will overstate cost.
For teaching, state whether watts came from a label or a measurement so classmates can reproduce the same 2.25 style check.
Common mistakes
- Treating hours as the total for the whole month instead of hours per day
- Forgetting the /1000 step and treating watt hours as kWh
- Entering 15 instead of 0.15 for a 15 cent rate
- Comparing one device cost to a full household bill without saying so
Daily hours discipline
Hours and days multiply. Five hours for 30 days is 150 operating hours, not 5. Write “5 hours/day × 30 days” in notes so nobody collapses the product into a single ambiguous hours field.
If a device runs only on weekdays, set days to the weekday count in the period rather than forcing 30 and hoping the story is clear.
Classroom drills
Hold 100 W, 5 hours, and 0.15. Compare 7 days versus 30 days. Energy scales from 3.5 kWh to 15 kWh and cost from 0.525 to 2.25. Then hold 30 days and raise watts to 200. Expect energy and cost to double.
Ask students to compute kWh first, then cost, as two separate lines. That habit prevents rate and energy mixups.
Reporting habits
Write “15 kWh × 0.15 = 2.25 for 100 W at 5 h/day over 30 days” rather than a bare currency figure. The full chain is what makes the estimate auditable.
When comparing two devices, keep days and rate identical so only watts or hours explain the difference.
Rate decimals and currency
Utility rates are often quoted in cents. Convert to dollars per kWh before entry. Fifteen cents is 0.15. Entering 15 multiplies cost by 100 and produces nonsense for a small appliance.
Currency symbols are display choices. The arithmetic is still energy × rate.
Always on loads
Set hours to 24 when a device never turns off. A 100 W always on load for 30 days uses ((100 × 24 × 30) / 1000) = 72 kWh. At 0.15 that is 10.80. Contrast that with the default 5 hour pattern so learners feel hours in their bones.
Standby modes need their own watt estimate. Do not assume zero without evidence.
Comparing two devices fairly
Keep days and rate identical when you compare two watt ratings. Change only watts or only hours so the cost gap has a clear cause. Mixing rate changes with watt changes in the same demo hides which lever moved the bill.
A short worksheet can list Device A at 100 W and Device B at 60 W under the same 5 hour and 30 day pattern. Energy becomes 15 kWh versus 9 kWh. At 0.15 the costs are 2.25 versus 1.35.
Limitations
No tiered rates, no fixed fees, no power factor, and no whole home aggregation. Output is one device path: ((watts × hours × days) / 1000) × rate.