Estimate what an appliance costs to run
Turn watts, hours, and your kWh rate into a running-cost figure — for one device or a comparison.
Tools used in this guide
The job
You want to know whether leaving a device on actually matters, or which of two appliances costs more to run. The calculator multiplies watts by hours and days to get kilowatt-hours, then applies your rate.
How to do it
Find the wattage on the nameplate, the manual, or a plug-in watt meter — a real measurement beats the label for anything that cycles. Enter average hours per day and how many days. For the rate, divide your last bill total by the kWh used; that folded-in rate is more honest than the headline tariff.
Worked example
A 100 W device on for 8 hours a day uses 24 kWh over 30 days. At ¥31 per kWh that is about ¥744 a month — worth knowing before assuming a standby gadget is free.
Limits
The math assumes the entered watts are drawn constantly. Fridges, heaters, and air conditioners cycle on and off, so nameplate watts overstate their average — measure or check real-world figures. Tiered pricing, taxes, fuel adjustments, and fixed monthly charges are outside this number.