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Electricity cost calculator

Electricity cost of running an appliance per hour, day, month and year, from its wattage, hours of use and your price per kWh (unit).

Drafted with AI writing toolsUpdated Checked examples: 5

h
days
$
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Cost per month
$
Cost per month: $27.00
Decimal places: 2; Nearest, ties to even
Cost per year
$324.00
Cost per day of use
$0.90
Cost per hour
$0.2250
Energy per day of use
6kWh
Energy per month
180kWh
Energy per year
2,160kWh
The rated power is a maximum: fridges, ACs and heaters cycle on and off, so their real use is usually lower. Tariffs with slabs, fixed charges or time-of-day rates will differ.

Running it 4 h a day uses 6 kWh per day of use: 180 kWh a month, about $27.00 at $0.150 per kWh, or $324.00 a year.

Monthly cost with the same use (4 h a day, 30 days)

LED bulb 9 W$0.16Laptop 60 W$1.08Fridge 150 W$2.70Yours: 1.5 kW$27.00AC 1.8 kW$32.40
How it's calculated S

  1. Power in kilowatts

    P=1,500 W×11000=1.5 kWP = \frac{1{,}500\ \mathrm{W}\times 1}{1000} = 1.5\ \mathrm{kW}

  2. Energy per day

    Eday=1.5 kW×4 h=6 kWhE_{\text{day}} = 1.5\ \mathrm{kW}\times 4\ \mathrm{h} = 6\ \mathrm{kWh}

  3. Energy per month and year

    Emonth=6×30=180 kWh,Eyear=12×Emonth=2,160 kWhE_{\text{month}} = 6\times 30 = 180\ \mathrm{kWh},\quad E_{\text{year}} = 12\times E_{\text{month}} = 2{,}160\ \mathrm{kWh}

  4. Cost

    costmonth=180×0.15=27,costyear=324\text{cost}_{\text{month}} = 180\times 0.15 = 27,\quad \text{cost}_{\text{year}} = 324

    Money is shown rounded to the currency's minor unit; the year is 12 months of the pattern you entered.

About the electricity cost calculator

The calculator turns an appliance's power rating into energy and money. Energy per day is the power in kilowatts times the hours of use, in kilowatt-hours (kWh); the monthly figure multiplies that by the days used, the yearly one by 12 months, and each is priced at your tariff per kWh. A kWh is the "unit" counted on electricity bills.

People use it to compare appliances or to see what a heater or air conditioner adds to a bill. The default, a 1.5 kW heater running 4 hours a day for 30 days at 0.15 per kWh, uses 6 kWh a day and 180 kWh a month: 27.00 a month and 324.00 a year.

The rated wattage is a maximum. Fridges, air conditioners and heaters with thermostats cycle on and off, so their real use is lower, and tiered tariffs, standing charges and time-of-day rates change the final bill.

How to calculate electricity cost

Multiply the appliance's power in kilowatts by the hours it runs to get kilowatt-hours (kWh), then multiply by your price per kWh. A 100 W lamp used 5 hours a day uses 0.1 × 5 = 0.5 kWh a day, or 15 kWh over 30 days. At 0.12 per kWh that costs 1.80 a month.

One kWh is the "unit" counted on electricity bills: 1,000 W for one hour, or 3.6 million joules. A 100 W lamp takes 10 hours to use one kWh, and a 2,000 W heater takes 30 minutes.

The electricity cost formula step by step

  1. Divide the power in watts by 1,000 to get kilowatts.
  2. Multiply by the hours of use per day to get kWh per day.
  3. Multiply by the days of use to get kWh per month.
  4. Multiply by the price per kWh to get the cost.

In one line: cost = watts × hours × days × price ÷ 1,000. The U.S. Department of Energy's appliance energy guide uses the same method, and its educational module explains wattage, hours and kilowatt-hour use.

Worked examples

Energy and cost are for one month of the stated use. The prices are examples; use the rate per kWh on your own bill.

Power (W)CountHours/dayDaysPrice/kWhEnergy (kWh)Cost
150014300.1518027
10015300.12151.8
910630816.2129.6
200010300.1500
  • 1,500 W heater, 4 hours a day: 1.5 × 4 = 6 kWh a day and 180 kWh a month, which costs 27.00 at 0.15 per kWh. Each hour of use costs 1.5 × 0.15 = 0.225.
  • Ten 9 W LED bulbs, 6 hours a day: 10 × 9 × 6 ÷ 1,000 = 0.54 kWh a day and 16.2 kWh a month, or 129.60 at a tariff of 8 per kWh.
  • An appliance left unused costs nothing to run, although any fixed charge on the bill still applies.

Monthly and yearly electricity cost

The yearly figure is twelve repeats of the month you describe. With 30 days of use a month that is 360 days, so the heater's year comes to 12 × 180 = 2,160 kWh and 324.00.

For an appliance used every day of the year, multiply the daily energy by 365 instead: 6 × 365 = 2,190 kWh, or 328.50 at 0.15 per kWh.

Why the real bill can differ

  • Appliances that cycle. Fridges, air conditioners and thermostat-controlled heaters switch on and off, so their average draw is below the label rating. A plug-in energy meter or the annual kWh on an energy label gives a better figure.
  • Standby power. A device drawing 5 W for the 20 hours a day it sits idle adds 0.1 kWh a day, or 3 kWh over 30 days. Work it out separately and add it to the active use.
  • Tariff structure. Slab or tiered prices, time-of-day rates, fixed charges and taxes all change the total. For a tiered tariff, price each block of kWh at its own rate and then add the fixed charges.

Common mistakes

  • Forgetting to divide by 1,000. Watts times hours gives watt-hours: 1,500 W for 4 hours is 6,000 Wh, which is 6 kWh.
  • Entering minutes as hours. Fifteen minutes is 0.25 h, so a 2,000 W hair dryer used 15 minutes a day uses 0.5 kWh a day.
  • Mixing currencies. No exchange rate is applied, so enter the price per kWh in the currency you want the cost shown in.

Input formats and units

  • Power accepts W, kW or hp. A rating given only in amps converts by multiplying by the supply voltage: 5 A at 230 V is 1,150 W.
  • Hours per day run from 0 to 24 as decimal hours, and days per month from 0 to 31.
  • The number of appliances multiplies identical devices that share the same hours of use.

To get watts from volts and amps, use the Ohm's law calculator. For energy in joules or mechanical power, use the work, energy and power calculator; for charging an electric car, the EV charging cost calculator.

Worked examples

1.5 kW heater, 4 h a day, 30 days at 0.15

Appliance power
1500 W
Hours used per day
4 h
Days used per month
30 days
Price per kWh (unit)
0.15
Energy per day of use
6 kWh
Energy per month
180 kWh
Cost per month
27.00
Cost per year
324.00

Checked against: DOE method: 1500 W × 4 h ÷ 1000 = 6 kWh/day; × 30 × 0.15 = 27.00; × 12 = 324.00

100 W bulb, 5 h a day at 0.12

Appliance power
100 W
Hours used per day
5 h
Days used per month
30 days
Price per kWh (unit)
0.12
Energy per month
15 kWh
Cost per month
1.80

Checked against: 0.1 kW × 5 × 30 × 0.12

Two 1 hp motors, 8 h on 22 working days at 0.20

Appliance power
1 hp
Hours used per day
8 h
Days used per month
22 days
Price per kWh (unit)
0.20
Number of appliances
2
Energy per day of use
11.9312 kWh
Cost per month
52.50
Cost per year
629.97

Checked against: 2 × 745.69987 W × 8 h = 11.9312 kWh/day; × 22 × 0.20 = 52.4973

Ten 9 W LED bulbs, 6 h a day at ₹8

Appliance power
9 W
Hours used per day
6 h
Days used per month
30 days
Price per kWh (unit)
8
Number of appliances
10
Energy per month
16.2 kWh
Cost per month
129.60

Checked against: 10 × 9 W × 6 h × 30 = 16.2 kWh; × 8 = 129.60

Questions

How do I calculate the cost of running an appliance?

Multiply the power in kilowatts by the hours of use to get kilowatt-hours, then multiply by the price per kWh. A 2,000 W (2 kW) heater used for 3 hours uses 6 kWh; at 0.15 per kWh that costs 0.90. The US Department of Energy gives the same method: watts × hours used per day ÷ 1,000 = daily kWh.

What is a unit of electricity?

One unit is one kilowatt-hour (kWh): 1,000 watts used for one hour, which is 3.6 megajoules of energy. A 100 W bulb takes 10 hours to use one unit and a 2 kW kettle takes 30 minutes. Bills in the UK, India and many other countries count units, and the price per unit is the tariff to enter here.

How do I find an appliance's wattage?

Look for the rating label on the back or base of the appliance, or in its manual; it lists watts or kilowatts. If it gives only amps, multiply by the supply voltage, as the US Department of Energy suggests: 5 A at 230 V is 1,150 W. A plug-in energy meter measures real consumption, including standby power and thermostat cycling.

How much electricity does a fridge use?

Less than its rating suggests, because the compressor switches on and off. The US Department of Energy suggests dividing the hours a fridge is plugged in by three to estimate its full-power running time, so a 150 W fridge runs about 8 hours a day: 1.2 kWh a day, or 36 kWh in a 30-day month. The kWh-per-year figure on its energy label is a better guide when you have it.

About this calculator

Eday=P×h×n1000 kWh,costmonth=Eday×d×tariff,costyear=12×costmonthE_{\text{day}} = \frac{P \times h \times n}{1000}\ \text{kWh},\quad \text{cost}_{\text{month}} = E_{\text{day}} \times d \times \text{tariff},\quad \text{cost}_{\text{year}} = 12 \times \text{cost}_{\text{month}}

Sources

  1. Virginia Cooperative Extension 2901-9014 — Estimating appliance and home electronic energy use
  2. BIPM SI Brochure (9th ed.), Table 8 — the kilowatt-hour is 3.6 MJ

Checked against references

This calculator includes 5 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.

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