This guide is for anyone who wants to work out what an appliance costs to run, students learning about power and energy, and anyone planning where to save electricity.
“I cannot remember the formula.” “What is the difference between W and kWh?” “Where does the 31 yen per kWh guideline come from?” Electricity cost becomes straightforward once you can separate power (W, kW) from energy (kWh) and find your own rate. This guide covers the calculation, the rate, domestic examples, why bills differ from the estimate, and how to check your work with a free calculator.
The formula
| Quantity | Formula | Unit |
|---|---|---|
| Energy used | Power (kW) × time (h) | kWh |
| Cost per use or per day | Energy (kWh) × rate (per kWh) | currency |
| Cost per month | Daily energy × days × rate | currency |
A 1200 W (1.2 kW) hair dryer used for 0.2 h (12 minutes) uses 1.2 × 0.2 = 0.24 kWh. At 31 per kWh the cost is 0.24 × 31 = 7.44, or 223.2 over 30 days (7.2 kWh).
W is power (the rate of use) and kWh is energy (the amount used). Running a 1000 W appliance for one hour uses 1 kWh, and the rate is quoted per kWh.
W, kW, kWh and J
| Unit | Meaning | Conversion |
|---|---|---|
| W (watt) | Power: energy per second | 1000 W = 1 kW |
| Wh (watt hour) | Energy: 1 W for 1 hour | 1000 Wh = 1 kWh |
| kWh (kilowatt hour) | Energy: the unit used for tariffs | 1 kWh = 3.6×10⁶ J |
| J (joule) | Energy (work) | 1 Wh = 3600 J |
1 kWh = 3.6×10⁶ J = 3.6 MJ follows from the definition of the units (NIST conversion factors). In heat units, 1 kWh ≈ 860 kcal (1 cal = 4.184 J), so a 1 kWh electric heater delivers roughly 860 kcal of heat. See the heat guide for the heat calculations.
Finding your rate: what 31 yen/kWh means
Your rate depends on your utility, plan and usage. The 31 yen/kWh seen in catalogues and articles is the guideline rate set by the Japan Electrical Fair Trade Council, revised from 27 to 31 on 22 July 2022 (JFTC FAQ). It is used as a common yardstick for comparing appliances.
Your own rate is on your bill or utility account as the energy charge per kWh. Note that a bill is not only the energy charge: it also includes the standing charge, fuel-cost adjustment, the renewable energy levy and taxes (ANRE). This guide and the calculator estimate the energy charge only.
Example 1: a 1200 W hair dryer for 12 minutes a day
- Power: 1200 W = 1.2 kW
- Energy per day: 1.2 kW × 0.2 h = 0.24 kWh
- Cost per day: 0.24 × 31 = 7.44
- Per 30 days: 7.2 kWh, 223.2
- Per year: about 2,716
Small daily amounts add up: a few units a day becomes thousands over a year.
Example 2: a 600 W air conditioner for 8 hours a day
- Power: 600 W = 0.6 kW
- Energy per day: 0.6 × 8 = 4.8 kWh
- Cost per day: 4.8 × 31 = 148.8
- Per 30 days: 144 kWh, 4,464
- Per year at the same usage: about 54,300
An air conditioner’s actual draw depends heavily on the set temperature, outdoor temperature and room size. Treat this as “what if it averaged 600 W for 8 hours”. If the catalogue gives annual energy consumption in kWh, that is closer to real use.
Example 3: a refrigerator is not “24 hours × rated power”
Even if the label says 150 W, the compressor cycles. Using 150 W × 24 h = 3.6 kWh/day (111.6/day) is an upper estimate.
A better figure comes from the annual energy consumption. For an example refrigerator rated at 200 kWh/year:
- Per year: 200 × 31 = 6,200
- Per month: 6,200 ÷ 12 ≈ about 517
- Per day: 200 × 31 ÷ 365 ≈ 17
For appliances that run continuously, start from annual energy and divide.
Why the estimate differs from the bill
- Standing charge, fuel-cost adjustment, renewable levy and taxes are not included
- Multiple appliances: the estimate covers one appliance, while the bill covers the whole home
- Standby power: TVs, recorders and similar devices add a little all day
- Seasonal variation: air conditioners and heaters swing strongly by month
- Rate differences: your plan’s rate may not be the 31 guideline value
Finding an appliance’s wattage
- The rating label: look for “power consumption” on the back or bottom of the appliance
- Manual or catalogue: some list power per operating mode
- From current and voltage: for a single-phase 100 V appliance, P = V × I gives 10 A × 100 V = 1000 W. AC motors draw less than this because of the power factor (Ohm’s law guide)
Check your work with the calculator
The Tools Hub electricity calculator mirrors the examples above.
Example 1
With the type set to "Electricity cost (power and time)", press "Hair dryer 1200 W, 12 min/day" to get 223.2 per month, 7.44 per day, 2,716 per year and 7.2 kWh per month, with the steps "P = 1,200 W = 1.2 kW" and "Energy per day = 1.2 kW × 0.2 h = 0.24 kWh".
Change the rate and hours
The rate field can be changed from 31 to your own tariff. Doubling the hours doubles the cost. "Air conditioner 600 W, 8 h/day" gives 4,464 per month, and "Refrigerator 150 W, 24 h/day" gives 3,348 per month as an upper estimate. Power can also be entered from current A and voltage V.
Convert energy units
Switch the type to "Energy unit conversion" to see 1 kWh = 3.6×10⁶ J = 1,000 Wh = 860.4 kcal. This is useful when you have energy in Wh or kWh and want to compare it with heat in joules or kcal.
Tool used in this guide
Electricity Cost & Energy Calculator
Turn power (W, or A × V) and usage time into kWh and cost per day, month and year, with an editable rate and energy unit conversion.
Applicability and limits
- The estimate covers the energy charge only; standing charges, fuel-cost adjustments, renewable levies and taxes are excluded
- Appliance power varies with operating conditions, especially for air conditioners, refrigerators and washing machines. For continuously running appliances, use the annual energy consumption
- A month is taken as 30 days and a year as 365 days; real billing periods differ
- Rates change with plan, usage and season — check your bill
- Energy conversions use 1 kWh = 3.6×10⁶ J, 1 Wh = 3600 J and 1 cal = 4.184 J
FAQ
What is the difference between kW and kWh?
kW is power, the rate of energy use; kWh is energy, the amount used. A 1000 W appliance running for one hour uses 1 kWh. Tariffs are quoted per kWh, so convert to kWh (power in kW × hours) before multiplying by the rate.
Where does 31 yen/kWh come from?
It is the guideline electricity rate set by the Japan Electrical Fair Trade Council, revised to 31 yen/kWh including tax on 22 July 2022. It is used for comparing appliances, and differs from your own tariff.
Why does the estimate differ from my bill?
The estimate is the energy charge only and excludes the standing charge, fuel-cost adjustment, renewable levy and taxes. A household bill also includes every appliance and standby power, and your rate may differ from the guideline.
How do I estimate an air conditioner accurately?
Check the annual energy consumption (kWh/year) from the catalogue or manufacturer, then divide by 12 for a month or 365 for a day and multiply by your rate. Actual use varies with temperature settings and outdoor conditions, so treat it as a guide.
Can I estimate standby power?
If you know the standby wattage, enter it as the power, 24 h per day and 30 days per month. Real standby power varies by device; use the manual or a power meter for accurate values.
How many kcal is 1 kWh?
1 kWh = 3.6×10⁶ J ≈ 860 kcal using 1 cal = 4.184 J. This helps when comparing electrical energy with heat.
How this guide was checked
On 6 October 2026 the definitions and conversions were checked against the sources above, and the example values were recalculated by hand (0.24 kWh/day giving 7.44/day and 223.2/month, 4.8 kWh/day giving 148.8/day and 4,464/month, 150 W × 24 h giving 3.6 kWh/day, 200 kWh/year giving about 517/month, and 1 kWh = 3.6×10⁶ J = 860.4 kcal). The values are teaching examples, not measurements. The calculator inputs and outputs were also verified.
