Enter the values
How to use the electricity cost calculator, and how household bills work
"What does leaving this on actually cost per month?" is answerable from three numbers: the appliance's power draw in watts, how long it runs, and your unit price. This tool shows cost and energy use (kWh) per hour, day, month and year.
How to use it
The wattage is printed on the rating plate on the back or side of the appliance, and in the manual. Enter the daily running time and your unit price. If you do not know the price, around 31 yen/kWh is a reasonable Japanese household figure — though the real rate depends on your supplier, plan and time of day, so the "energy charge" line on your bill is the reliable source.
W, kW and kWh
These are easy to mix up. Watts measure how fast energy is being used; kilowatt-hours measure how much has been used in total. A 1000 W (1 kW) appliance running for an hour consumes 1 kWh, and bills are essentially kWh multiplied by the unit price. A 500 W device for two hours and a 1000 W device for one hour cost exactly the same.
Typical appliance figures
An air conditioner cooling a small room draws roughly 400–600 W while running, a microwave 1000–1400 W, a kettle about 1200 W, a hairdryer about 1200 W, and a fridge uses 250–400 kWh a year. A high-wattage appliance used briefly costs little, while a modest one running around the clock — the fridge — adds up to a large annual figure.
Standby power and contracted capacity
Surveys by Japan's Agency for Natural Resources and Energy put standby power at about 5% of household electricity use. Unplugging rarely used devices, or grouping them on a switched power strip, is a genuine saving. Most Japanese suppliers also charge a base rate tied to your contracted amperage on top of usage, so lowering the contracted amperage lowers the fixed charge — at the cost of how much you can run simultaneously.
Where the savings actually are
In rough order of impact: review the things that run longest (fridge, lighting, air conditioning); replace old appliances with efficient models; and relax the thermostat by a degree or two. To judge a replacement, divide the price difference by the annual running-cost saving to get a payback period. See also the compound interest calculator and the percentage calculator.
Frequently asked questions
How is the electricity cost calculated?+
A month is treated as 30 days and a year as 365 days.
What unit price should I enter?+
If unknown, a rough default is provided, but replace it with the rate on your bill.
Can I estimate standby power?+
Why does it differ from my actual bill?+
This tool estimates only the usage-based energy charge.
Are my inputs stored?+
・Energy (kWh) = power (W) × hours ÷ 1000
・Cost = kWh × unit price per kWh. A month is counted as 30 days, a year as 365.
・The default price (about 31 yen/kWh) is a rough Japanese household average and varies by contract, plan and time of day; check the energy charge line on your bill.
・Agency for Natural Resources and Energy, Japan. Survey on household standby power: standby power is about 5% of household electricity use
This site does not represent any electricity provider or other organization and does not guarantee the accuracy of the results. Actual charges vary with base fees, adjustments and tiered rates. Check your provider's tariff and bill for exact amounts.