A method you can check
Watts and kilowatt-hours answer different questions. A watt (W) describes the rate at which electrical energy is used at a moment. A kilowatt-hour (kWh) measures the amount of energy used over time. Electricity bills commonly charge for kWh, not for the appliance's watt rating by itself.
The conversion in one line
Convert watts to kilowatts by dividing by 1,000, then multiply by the number of hours the device uses that power:
A 200-watt device operating for five hours at a constant 200 W would use 1 kWh. A 1,000-watt device operating for one hour would also use 1 kWh. The power is different, but the total energy is the same.
From energy to an estimated charge
Multiply the kWh by the energy price per kWh. If 30 kWh are used and the rate is 0.20 currency units per kWh, the energy charge is 6 currency units. The example uses an arbitrary rate; your actual bill can have fixed fees, taxes, tiers, or time-of-use prices that change the total.
Why estimates can differ from actual use
The simple calculation assumes constant power. Many appliances do not operate at a constant load. A refrigerator cycles its compressor. A laptop may draw more power under a heavy workload than while idle. An air conditioner can run at variable speed. For such equipment, use an average input estimate, an energy-per-cycle specification, or a meter reading if available.
A practical checklist
- Find the electrical input rating.
- Estimate typical hours, rather than the longest possible day.
- Account for automatic cycling when it is significant.
- Use your actual tariff and billing period.
- Compare the result with the bill as a reasonableness check.
Try the Electricity Cost Calculator for one device or the Home Energy Planner for a list of devices.
Related resources
Continue with the Electricity Cost Calculator, the Home Energy Planner, or the Appliance Comparison Calculator depending on the question you are trying to answer.
References and further reading
The U.S. Energy Information Administration explains the distinction between watts, watt-hours, and kilowatt-hours in its guide to measuring electricity. Its kilowatt-hour definition is useful when checking the unit on a bill.
These sources explain general units; they do not set your local tariff. Use your own utility bill for the price per kWh and check whether the bill includes fixed charges or tiered rates.