Electricity Cost Calculator
Monthly kWh and cost for any device from watts, hours, and your rate.
What it does
Electric bills multiply two things: how fast a device drinks power, in watts, and how long it runs. A thousand watt hours make one kilowatt hour, the unit on the bill. Price per kWh turns that energy into money.
The tool above takes watts, daily hours, and your rate. It returns daily and monthly kWh with the monthly cost, so heaters, PCs, and old bulbs can be compared honestly.
Standby loads deserve respect because they never sleep. A 10 watt box idling all month burns over 7 kWh, and a house holds dozens of such boxes. Big short loads like kettles look scary in watts but sip in time.
Rates move a lot. US residential prices span roughly ten to forty cents by state and plan, which doubles or halves every answer here. Read your exact rate off the bill, including delivery charges when comparing.
How to use
- Read watts off the device label or plug manual.
- Enter real daily hours, averaging weekends in.
- Enter your full per kWh rate from the bill.
- For always on gear, enter 24 hours and watch the cost appear.
Formula
- watts
- Power draw while running. Heating elements run near label; fridges cycle.
- hours
- Running hours per day, averaged over the week.
- 1000
- Watts per kilowatt. Dividing converts W to kW.
- rate
- Your price per kWh including delivery, from the bill.
Cycling devices like fridges and heaters with thermostats run part of the time, so enter effective on hours, not plugged hours. A fridge labeled 150 W might average 50 W across the day.
Worked example
Example: 1500 W heater, 4 hours a day, $0.17 per kWh
Space heater 1500 W, evening use 4 hours, average US rate.
- Daily: 1500 x 4 / 1000 = 6 kWh.
- Monthly: 6 x 30 = 180 kWh.
- Cost: 180 x $0.17 = $30.60 per month.
- Year: $30.60 x 12 = $367.20 for that one heater habit.
Reference table
| Device | Typical use | Monthly cost |
|---|---|---|
| LED bulb 9 W, 5 h/day | 1.35 kWh | $0.23 |
| Old bulb 60 W, 5 h/day | 9 kWh | $1.53 |
| Gaming PC 400 W, 4 h/day | 48 kWh | $8.16 |
| Space heater 1500 W, 4 h/day | 180 kWh | $30.60 |
| Old fridge, extra, 100 W avg, 24 h | 72 kWh | $12.24 |
Limits
- Assumes steady draw. Cycling and variable loads need averaged watts.
- Uses a 30 day month and flat rate. Tiered and time of use plans vary.
- Ignores taxes and fixed fees, which bills add on top.
- Startup surges in motors barely register on energy but matter for circuits.
Common mistakes
- Confusing watts with kilowatt hours. Watts are speed, kWh are distance.
- Pricing with the supply rate while ignoring delivery charges.
- Counting plugged hours for thermostats instead of on hours.
- Missing 24 hour standby loads that dwarf visible use.
- Comparing states without comparing rates.
Try it yourself
Quick checks. Answers sit tucked under each question.
A 100 W bulb runs 6 hours a day. Daily kWh?
100 x 6 / 1000 = 0.6 kWh a day.
That bulb for a month at $0.20 per kWh?
0.6 x 30 = 18 kWh. 18 x $0.20 = $3.60.
Questions
What is a kWh in plain words?
Running 1000 watts for one hour. A 100 W bulb needs ten hours to burn one kWh.
Why is my bill higher than the math?
Fixed fees, taxes, tiered rates, and loads you forgot: water heater, dryer, and standby gear.
What uses the most power at home?
Heat and hot water lead: space heaters, water heaters, dryers, and ovens, all above 1000 W.
Do phone chargers matter?
Barely. A phone charge holds about 0.015 kWh, under a cent. The old set top box beside the TV matters far more.
How do I find a device watts?
Read the label plate, check the manual, or measure with a plug meter for the true draw.
Does unplugging save real money?
Yes for warm standby boxes. A power strip collecting TV gear can save several dollars a month.