Watts to kWh Calculator
Calculate energy consumption and electricity cost from watts and usage hours.
How do you convert watts to kWh?
Multiply watts by hours of use, then divide by 1000: kWh = (W × h) ÷ 1000. A 1500 W heater running 3 hours uses (1500 × 3) ÷ 1000 = 4.5 kWh. At an electricity price of $0.15 per kWh, that run costs about $0.68.
Watts to kWh Conversion Table
| Power | 1 h | 5 h | 10 h | 24 h |
|---|---|---|---|---|
| 100 W | 0.10 kWh | 0.50 kWh | 1.00 kWh | 2.40 kWh |
| 500 W | 0.50 kWh | 2.50 kWh | 5.00 kWh | 12.0 kWh |
| 1000 W | 1.00 kWh | 5.00 kWh | 10.0 kWh | 24.0 kWh |
| 1500 W | 1.50 kWh | 7.50 kWh | 15.0 kWh | 36.0 kWh |
Watts to kWh Formula
To calculate kilowatt-hours, multiply the power consumption in watts by the number of hours the device is used, then divide the result by 1,000 to convert from watt-hours to kilowatt-hours.
Reverse formula: W = (kWh × 1,000) ÷ h
to kWh to Watts Converter
Worked Example:
A 900 W air conditioner running 6 hours a day for 30 days:
- Daily kWh = (900 × 6) ÷ 1,000 = 5.4 kWh
- Monthly kWh = 5.4 × 30 = 162 kWh
Common Appliance Energy Use
| Appliance | Watts | Daily Hours | Daily kWh | Monthly kWh |
|---|---|---|---|---|
| Refrigerator (modern) | 150 W | 24 h | 3.60 kWh | 108 kWh |
| Air conditioner (small) | 900 W | 8 h | 7.20 kWh | 216 kWh |
| Electric heater | 1,500 W | 6 h | 9.00 kWh | 270 kWh |
| Washing machine | 500 W | 1 h | 0.50 kWh | 15 kWh |
| Dishwasher | 1,200 W | 1 h | 1.20 kWh | 36 kWh |
| LED TV (55") | 80 W | 5 h | 0.40 kWh | 12 kWh |
| Desktop computer | 200 W | 8 h | 1.60 kWh | 48 kWh |
| LED bulb | 10 W | 8 h | 0.08 kWh | 2.4 kWh |
| Phone charger | 5 W | 2 h | 0.01 kWh | 0.3 kWh |
| Electric oven | 2,400 W | 1 h | 2.40 kWh | 72 kWh |
High Watts vs. Long Hours: What Drives Your Bill?
To understand electricity usage, you have to look at both the wattage and the running time. A high-wattage device used briefly can cost less than a low-wattage device left on continuously. Here are typical household examples:
- Space Heater (1,500 W) run for 8 hours:
(1,500 × 8) ÷ 1,000 = 12 kWh. This is a massive energy draw; running it daily for a month uses 360 kWh, which can easily add $50–$80 to a monthly bill. - Microwave (1,200 W) used for 15 minutes (0.25 h):
(1,200 × 0.25) ÷ 1,000 = 0.3 kWh. Despite the high wattage, short burst usage makes microwaves very energy efficient per meal. - Desktop PC & Monitor (250 W) left on 24/7:
(250 × 24) ÷ 1,000 = 6 kWh per day (180 kWh per month). Always-on loads ("vampire" or idle loads) quietly consume huge amounts of energy over time. - LED Light Bulb (10 W) left on 24/7:
(10 × 24) ÷ 1,000 = 0.24 kWh per day. LED efficiency means even leaving lights on accidentally has a minimal impact compared to heating or cooling.
Frequently Asked Questions
kWh = (W × hours) ÷ 1,000. Multiply the device's power rating in watts by the number of hours it runs, then divide by 1,000 to convert watts to kilowatts. Example: a 1,500 W heater running for 4 hours = (1,500 × 4) ÷ 1,000 = 6 kWh.
(1,000 × 24) ÷ 1,000 = 24 kWh per day. At a typical US rate of $0.16/kWh, that is 24 × $0.16 = $3.84 per day, or approximately $115.20 per month.
Energy = (1,500 × 8) ÷ 1,000 = 12 kWh. At $0.16/kWh: 12 × $0.16 = $1.92. Monthly (8 h/day): 12 × 30 = 360 kWh, costing $57.60.
Watts (W) measure the rate of power consumption — how fast energy is used. Kilowatt-hours (kWh) measure the total energy consumed over time. A 100 W bulb uses 100 W at any instant, but running it for 10 hours consumes (100 × 10) ÷ 1,000 = 1 kWh of energy. kWh is what your electricity bill charges.
Does a 1,500W space heater use 1.5 kWh?
Only if you run it continuously for exactly one hour. If you run it for 4 hours, it uses 6 kWh. If it runs on a thermostat and the heating element is only on 50% of the time over 4 hours, it uses 3 kWh.