Enter an appliance's power in watts and how long it runs to get the energy it uses in kilowatt-hours per day, week, month and year. Add your electricity price to see what it costs.
Last updated:
W
From the appliance label. For kW, multiply by 1,000.
h
%
Share of the time it draws full power. 100% for most devices; about 30–50% for fridges and thermostatic heaters.
$/kWh
From your bill. Leave blank to skip cost.
Enter your values to see the result.
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How to use the Watts to kWh Calculator
Enter the power in watts from the appliance label (and how many appliances).
Enter the hours of use and choose whether that is per day, per week, per month or in total.
Set the duty cycle below 100% for devices that cycle on and off, like fridges.
Add your electricity price per kWh to see the cost, or leave it blank.
Compare with the typical appliances table and copy or download the results.
Watts to kWh formula
Watts measure power, the rate at which energy is used. Kilowatt-hours measure energy, the amount used over time, which is what your utility bills you for. To go from one to the other you need the time:
kWh = watts × hours ÷ 1,000
Example: a 1,500 W space heater running 6 hours a day uses 1,500 × 6 ÷ 1,000 = 9 kWh per day. Over an average month (30.42 days) that is about 274 kWh, and at $0.17 per kWh it costs about $46.54 a month.
For several identical devices multiply by the number of devices. For appliances that switch on and off by themselves (refrigerators, freezers, heaters with a thermostat), multiply by the duty cycle, the share of time the compressor or element is actually running.
Quick reference: kWh for 1 hour of use
Power
kWh per hour
kWh per day (24 h)
kWh per month (24/7)
Cost per hour at $0.17
10 W
0.01
0.24
7.3
$0.002
60 W
0.06
1.44
43.8
$0.010
100 W
0.1
2.4
73.0
$0.017
250 W
0.25
6
182.5
$0.043
500 W
0.5
12
365.0
$0.085
1,000 W
1
24
730.0
$0.170
1,500 W
1.5
36
1,095.0
$0.255
2,000 W
2
48
1,460.0
$0.340
3,000 W
3
72
2,190.0
$0.510
5,000 W
5
120
3,650.0
$0.850
Tips for accurate numbers
Label watts are maximums. A TV or computer often draws less than its rated power, while a fridge's label may show the compressor's running power. A plug-in energy meter gives the real figure.
Amps and volts? Watts ≈ volts × amps for resistive loads like heaters and kettles (120 V × 12.5 A = 1,500 W).
Standby power adds up. A device drawing 5 W around the clock uses about 3.65 kWh a month (5 × 24 × 30.42 ÷ 1,000).
Your rate may vary. Many utilities charge tiered or time-of-use prices plus fixed fees, so use your average cost per kWh from a recent bill. The default of $0.17/kWh is only a placeholder close to the recent US residential average.
Frequently asked questions
How do you convert watts to kWh?
Multiply the power in watts by the hours of use and divide by 1,000: kWh = W × h ÷ 1,000. A 100 W device running 10 hours uses 100 × 10 ÷ 1,000 = 1 kWh.
How many kWh does a 1,500 watt heater use?
It uses 1.5 kWh for every hour it runs at full power. Running 6 hours a day, that is 9 kWh per day or about 274 kWh per month.
How many watts is 1 kWh?
A kWh is energy, not power, so it depends on time. 1 kWh is 1,000 watts for one hour, 100 watts for 10 hours, or 10 watts for 100 hours.
How do I calculate the cost of running an appliance?
Find its kWh for the period (watts × hours ÷ 1,000) and multiply by your price per kWh. At $0.17 per kWh, 9 kWh a day costs $1.53 a day.
Why is my real usage lower than watts × hours?
Many devices do not draw their full rated power all the time. Fridges, freezers, air conditioners and thermostatic heaters cycle on and off, and electronics often run below their maximum. Use the duty cycle field or measure with a plug-in energy meter.
How many days are in a month for this calculator?
Monthly figures use an average month of 365 ÷ 12 = 30.42 days, so 12 months add up exactly to one year.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.