How to Calculate Electricity Costs | The Formula from W and kWh for an Hour or a Month

There is only one formula for electricity cost

You can work out an appliance's electricity cost with the formula below. To estimate a single appliance, you don't need to worry about all the detailed items on your power company's bill.

Dividing by 1000 converts W into kW. kW multiplied by hours gives “kWh” (kilowatt-hours), the unit of energy that power companies bill in, and the usage on your meter reading slip is written in this unit too.

1. 1

   Convert W to kW

   A 1,200 W hair dryer is 1.2 kW. Appliances over 1,000 W are sometimes listed as “1.2kW” in catalogs.

2. 2

   Multiply by the time used to get kWh

   Ten minutes a day is 10 ÷ 60 = 0.167 hours. 1.2 kW × 0.167 h = 0.2 kWh. With appliances you use for minutes, converting to hours is where mistakes happen, so take care.

3. 3

   Multiply by the price

   At the guideline price of 31 yen, 0.2 kWh × 31 yen = 6.2 yen. That is the electricity cost of one use of the hair dryer.

4. 4

   Multiply by the number of days for a month

   Used every day, that is 6.2 yen × 30 days = 186 yen, or 2,232 yen a year. Even a high-wattage appliance costs little per month if you use it only briefly.

What price per kWh should you use?

The electricity costs printed in appliance catalogs and on store displays in Japan are calculated with the “guideline unit price for new electricity rates” of 31 yen/kWh (tax included) set by the Electric Home Appliance Fair Trade Council. It was revised from 27 yen to 31 yen in July 2022 and serves as a common yardstick for comparing appliances across manufacturers.

On the other hand, the “effective price”, the bill amount divided by usage, is almost always higher than 31 yen. Building up a month of 300 kWh with the rates of TEPCO Energy Partner's Juryo Dento B plan (30 A) gives a bill of 12,257 yen, or 40.86 yen per kWh (with a fuel cost adjustment of 0 yen). The base charge and the renewable energy levy are added on top.

The table is calculated for a 6–8 tatami mat air conditioner on cooling (400 W on average). With the same usage, calculating at 31 yen or at 40 yen makes a difference of over 10,000 yen a year. For comparing appliances, 31 yen is fine, but when you want to explain why this month's bill is what it is, recalculating with your own home's effective price makes the numbers match up. How to work out the effective price is covered in detail in the breakdown of electricity charges.

Quick chart: power consumption × hours of use

The electricity cost of running something for the time shown, at 31 yen/kWh. For a month, multiply by the number of days you use it.

The one figure worth remembering is “1,000 W for one hour costs 31 yen”. 500 W is half that and 100 W a tenth, so the rest follows by proportion. The 24-hour column is the daily cost of leaving something on. Even a small 30 W device costs about 670 yen a month if left on all the time.

Three pitfalls that throw the calculation off

The formula is simple, but plugging in the wrong numbers can make the result several times off. These three are the most common.

• Using the rated power consumption as is: the “rated power consumption” in a catalog is the maximum. An air conditioner rises to around 1,000 W just after starting, but drops to about 100–200 W once the room reaches the set temperature. A kotatsu (heated table) is listed at 500–600 W on high, but the thermostat switches it on and off, so it averages around 150 W. Calculating with the rating gives several times the real amount.
• Confusing output with power consumption: a microwave's “500 W” or “600 W” is its heating power (output); the power it draws from the outlet is around 1,200 W. Use the power consumption for electricity costs.
• Using “60 W equivalent” as is: an LED bulb labeled “60 W type” or “60 W equivalent” is as bright as a 60 W incandescent bulb, but actually uses about 8 W. Calculating with 60 W gives more than seven times the cost.

In other words, what decides accuracy is not the arithmetic but the estimate of how much power the appliance uses on average. The appliance list in the Electricity Cost Calculator starts from values close to the average while running, not the rating.

Small wattages add up over 24 hours × 365 days

Even at a low wattage, a device that runs 24 hours a day can cost more per year than you would expect. Standby power, which flows even when the device is switched off, can be worked out per year in the same way.

Removing one device with 1 W of standby power saves 272 yen a year. Rather than going round unplugging things, reviewing the efficiency of big devices that run 24 hours a day, like the refrigerator, saves more money. Meanwhile, charging a smartphone costs 18.6 yen a month even for two hours a day, so “leaving it charging runs up the bill” is hardly ever true.

Calculating with the tool, step by step

The Electricity Cost Calculator has four tabs: “Calculate by appliance”, “Calculate from wattage”, “Compare electricity costs” and “Look up from your bill”. To find the cost of a single appliance, follow these steps.

1. 1

   Set the price

   “Price per kWh” at the top of the screen defaults to 31 yen. Switch with the 27, 31, 35 and 40 yen buttons, or enter your own home's price worked out from your meter reading slip.

2. 2

   Choose an appliance or enter the power consumption

   Pick from 48 appliances and typical power consumption and daily hours of use are filled in. For appliances not on the list, enter the W on the “Calculate from wattage” tab. Either “1.2kW” or “1,200W” is understood.

3. 3

   Set the hours of use and days

   You can also enter minutes, like “30 min” or “1 h 30 min”. The number of days per month can be changed too, so you can calculate for a PC used only on weekdays.

4. 4

   Check with the formula

   Below the result, the formula “W ÷ 1000 × hours = kWh × price” is shown as is. Doing the same on a calculator gives the same number, so you can check each step.

Specific amounts for each appliance are in air conditioner electricity costs and a comparison of heating appliance costs. Whether your whole household's bill is high compared with the average can be checked in average electricity bills. The power used by resistors and LEDs in electronics projects is worked out the same way, with W = voltage × current. How to read the values of those parts is explained in how to read resistor color codes.

FAQ

What price per kWh should I use?

For comparing appliances, the guideline price of 31 yen/kWh used in catalogs is enough. To get closer to your actual bill, use the effective price: the bill amount on your meter reading slip divided by usage. Because base charges and the renewable energy levy are added, homes on a standard metered plan often come to around 40 yen, and the lower the usage, the higher it gets.

How much does it cost to run a 1000 W appliance for one hour?

31 yen at 31 yen/kWh. 1000 W ÷ 1000 × 1 hour = 1 kWh, multiplied by 31 yen. 1500 W is 46.5 yen and 500 W is 15.5 yen, in proportion. Thirty minutes is half, ten minutes a sixth.

How do I calculate the cost for a month?

Multiply the daily cost by the number of days you use it in a month. The daily cost is power consumption (W) ÷ 1000 × daily hours of use × price. For example, a 400 W air conditioner for 8 hours a day is 0.4 × 8 × 31 = 99.2 yen, or 2,976 yen for 30 days. For a year, multiply by 12, but for seasonal appliances like air conditioners, multiplying by the number of months you use it is closer to reality.

Can I calculate with the rated power consumption in the catalog?

For appliances whose output goes up and down while running, such as air conditioners, kotatsu and refrigerators, it comes out far too high. The rating is the maximum, and an air conditioner drops to a fraction of it once the set temperature is reached. For a refrigerator, multiplying the catalog's annual energy consumption (kWh/year) by the price is accurate. For appliances that run near maximum the whole time they are in use, like hair dryers and electric kettles, the rating gives about the right answer.

How much does standby power cost?

A device with 1 W of standby power costs 272 yen a year over 24 hours × 365 days (at 31 yen/kWh). Most recent appliances use less than 1 W, so unplugging them one by one saves only a few hundred yen a year. Reviewing how you use the big consumers, such as the refrigerator, air conditioner and dryer, is more effective than worrying about standby power.