Watts to kWh Calculator
Enter power, daily run time, and number of days to calculate energy use in kilowatt-hours (kWh).
If you have ever looked at an appliance label and wondered what that wattage means for your electric bill, this page is for you. A watt tells you the rate of power draw, while a kilowatt-hour (kWh) tells you the amount of energy consumed over time. Since electric companies charge by kWh, converting watts to kWh is one of the most practical energy calculations you can learn.
How to convert watts to kWh
The core formula is simple:
kWh = (Watts × Hours × Days) ÷ 1000
- Watts (W) = power of the device.
- Hours = how long it runs per day.
- Days = number of days you are measuring.
- Divide by 1000 because 1000 watts = 1 kilowatt.
Example: A 1500W heater running 2 hours/day for 30 days uses (1500 × 2 × 30) ÷ 1000 = 90 kWh.
Watts vs watt-hours vs kilowatt-hours
Watts (W)
Watts are an instant rate of energy use. A 60W bulb is drawing 60 watts while it is on.
Watt-hours (Wh)
Watt-hours measure actual energy used over time. If that same 60W bulb runs for 5 hours, it uses 300Wh.
Kilowatt-hours (kWh)
A kilowatt-hour is 1000 watt-hours. So 300Wh is 0.3kWh. This is the unit that appears on your electric bill.
Common appliance conversions
| Appliance | Power (W) | Usage | Monthly Energy (kWh) |
|---|---|---|---|
| LED TV | 100 | 4 hrs/day | 12 |
| Laptop | 65 | 8 hrs/day | 15.6 |
| Microwave | 1200 | 0.5 hrs/day | 18 |
| Space Heater | 1500 | 3 hrs/day | 135 |
| Window AC | 1000 | 6 hrs/day | 180 |
How to estimate electricity cost
After you calculate kWh, multiply by your electricity rate:
Cost = kWh × Rate
If a device uses 90 kWh in a month and your rate is $0.15 per kWh, your estimated cost is:
90 × 0.15 = $13.50
This calculator can do that automatically if you enter your rate.
Tips to reduce kWh consumption
- Replace old bulbs with LEDs.
- Turn off space heaters and AC when rooms are empty.
- Use smart plugs or timers for devices with long idle periods.
- Unplug chargers and electronics that draw standby power.
- Upgrade to energy-efficient appliances with lower wattage.
Frequently asked questions
Can I convert watts to kWh without time?
No. You need time (hours) because watts alone are a rate, not total energy.
Why divide by 1000?
Because utility billing uses kilowatts, and 1 kilowatt = 1000 watts.
Is this calculator accurate?
It is accurate for estimating. Real-world usage can vary due to cycling behavior, appliance efficiency, and voltage differences.
Bottom line
Converting watts to kWh is the fastest way to understand appliance energy use and estimate utility costs. Use the calculator above to compare devices, plan your monthly consumption, and make smarter energy decisions.