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Watts to kWh Calculator.

Turn watts and hours of use into kilowatt-hours and energy cost.

This watts to kWh calculator turns an appliance's power and running time into kilowatt-hours and cost. A 1,500 W heater used 3 hours a day for 30 days uses 135 kWh, about $22.95 at $0.17 per kWh. Enter the watts, hours and days below.

Updated

INTERACTIVE TOOL

01 / Enter values

Your result

135 kWh

Estimated cost: 22.95

Per day: 4.5 kWh

Average power: 1.5 kW

Cost per day: 0.77

QUICK ANSWER

kWh = watts × hours ÷ 1,000. A 1,500 W heater used 3 hours a day for 30 days uses 1,500 × 90 ÷ 1,000 = 135 kWh. Going the other way, average watts = kWh × 1,000 ÷ hours.

THE METHOD

How it works

Energy (kWh) = power (watts) × hours of use ÷ 1,000. Cost = kWh × electricity price per kWh.

PUT IT TO USE

Example calculation

A 1,500 W heater for 3 hours a day over 30 days uses 135 kWh; at 0.17 per kWh that costs 22.95.

QUICK REFERENCE

Typical appliance power

ApplianceTypical wattskWh per hour of use
LED bulb90.009
Laptop500.05
Television1000.1
Refrigerator (running)1500.15
Microwave1,0001.0
Space heater1,5001.5
Electric oven2,4002.4

What does kWh mean?

kWh means kilowatt-hour, the energy used by a device drawing 1,000 watts for one hour. It is the unit on electricity bills. A 100 W bulb takes 10 hours to use 1 kWh, while a 2,000 W heater uses 1 kWh in 30 minutes. One kWh equals 3.6 million joules.

A kilowatt-hour is the energy used by 1,000 watts for one hour. Watts measure power (rate), kWh measure energy (amount), which is what your utility bills you for.

How many kWh does a house use?

According to the U.S. Energy Information Administration, the average US residential customer bought 10,791 kWh of electricity in 2022, about 899 kWh a month. Louisiana homes used the most (14,774 kWh a year) and Hawaii the least (6,178 kWh). Add up your main appliances in the calculator to see where your own usage comes from.

In short

How to use the Watts to kWh Calculator

  1. Fill in “Power”, for example 100 watts.
  2. Fill in “Hours used per day”, for example 8 hours.
  3. Fill in “Number of days”, for example 30 days.
  4. Read the result. It updates as you type, so you can change any value and compare.

Quick reference

Worked examples

PowerHours used per dayNumber of daysElectricity priceResult
100 watts8 hours30 days0.17 per kWh24 kWh
1000 watts1 hours30 days0.17 per kWh30 kWh
3500 watts2 hours30 days0.17 per kWh210 kWh
10 watts24 hours365 days0.17 per kWh87.6 kWh
Screenshot of the Watts to kWh Calculator on this site with a worked example filled in
Screenshot: the Watts to kWh Calculator with the example values entered.

Accuracy

How we checked this

We compared 4 results from this calculator with values we worked out independently, using exact decimal arithmetic and the published definitions, on October 26, 2026. 4 of 4 matched to the precision shown.

InputIndependent valueThis pageMatch
1500 watts · 3 hours · 30 days135135 kWhYes
60 watts · 5 hours · 30 days99 kWhYes
150 watts · 24 hours · 1 days3.63.6 kWhYes
2000 watts · 0.5 hours · 7 days77 kWhYes

From the source

In 2022, the average annual amount of electricity sold to (purchased by) a U.S. residential electric-utility customer was 10,791 kilowatthours (kWh), an average of about 899 kWh per month.

Source: U.S. Energy Information Administration

Sources

GOOD TO KNOW

Frequently asked questions

How do I convert watts to kWh?

Multiply watts by hours of use and divide by 1,000.

How do I convert kWh to watts?

Multiply kWh by 1,000 and divide by the hours: 3 kWh over 2 hours is 1,500 W.

How much does it cost to run a 100 watt bulb?

A 100 W bulb on for 8 hours a day uses 0.8 kWh a day, or 24 kWh in 30 days. At $0.17 per kWh that is $4.08 a month. An LED giving the same light uses about 15 W, so about $0.61.

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Related guides

How Many kWh Does a House Use?How Many Watts Does a Refrigerator Use?