How Much Does It Cost to Leave a 100W Light Bulb On All Day?
A 100W bulb left on 24 hours a day for 30 days at $0.15/kWh costs about $1.08 per month.
How to use this tool
- Enter power, hours per day, number of days and electricity rate in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your total cost and the full breakdown beneath it.
See the true monthly cost of leaving lights on around the clock.
Frequently asked questions
- How do I calculate electricity cost?
- Multiply the appliance wattage by hours used per day and number of days, divide by 1000 to get kWh, then multiply by your electricity rate per kWh.