Electricity Cost Calculator
Estimate the running cost of appliances from wattage, usage time, quantity, and your own electricity rate. Compare daily, monthly, and annualized costs without sending your appliance data to a server.
Formula
watts x hours x days / 1000 x rate
Appliance details
Quick presets
How to read the result
The calculator converts appliance wattage into kilowatt-hours, then multiplies that energy use by your price per kWh. Use the rate shown on your electricity bill for the closest estimate.
Compare appliances
Try the same usage period with different wattage values to compare heaters, air conditioners, lighting, laptops, or standby devices before changing habits.
Privacy-focused
Inputs are calculated locally in the browser. No account is required, and the result does not depend on any external energy pricing API.
Common Appliance Monthly Examples
These examples use a 30-day month and the default JPY 31 per kWh rate. Adjust the fields above when your bill shows a different electricity rate or your appliance runs for a different number of hours.
Bedroom or desk lamp
LED bulb
¥42
9 W for 5 hours per day
Work-from-home day
Laptop
¥363
65 W for 6 hours per day
Summer cooling estimate
Air conditioner
¥6,696
900 W for 8 hours per day
Spot heating comparison
Electric heater
¥4,464
1200 W for 4 hours per day
How to Improve Your Estimate
Electricity use changes by mode, season, and thermostat behavior. Use this checklist before comparing appliances or deciding whether a usage habit is worth changing.
Use the measured wattage from the appliance label, manual, or a plug-in power meter.
Split time-of-use plans into separate morning, daytime, and night calculations.
Run high-use appliances separately before combining them into a monthly household estimate.
When to use annualized cost
Annualized cost is useful for devices that run all year, such as refrigerators, routers, dehumidifiers, aquariums, or standby equipment. For seasonal devices, compare the selected period total instead.
Frequently Asked Questions
What electricity rate should I enter?
Use the price per kWh from your bill. If your plan has time-of-use rates, run the calculator separately for each rate period and add the totals.
Why does wattage differ from real usage?
Many appliances cycle on and off or change power draw by mode. Use measured wattage from a smart plug when you need a tighter estimate.
Can I calculate monthly cost?
Yes. Set the period to 30 or 31 days. The annualized figure uses your average daily cost multiplied by 365.
How do I compare two appliances fairly?
Keep the same period and rate per kWh, then change only the wattage, quantity, or hours per day. That makes the cost difference easier to read.
Should standby power be included?
Include standby power for devices that stay plugged in all day. Use a low wattage value, 24 hours per day, and a 30-day period to estimate the monthly cost.