Watts measure power, kWh measures energy
A watt tells you the rate at which a device uses or transfers energy. A kilowatt is 1,000 watts. A kilowatt-hour records what happens when power is used over time.
1 kWh = 1,000 watts used for 1 hour. It could also be 500 watts for 2 hours, or 2,000 watts for half an hour.
How to convert watts to kWh
Divide watts by 1,000 to get kilowatts, then multiply by hours. A 1,500 W appliance used for 20 minutes (0.333 hours) uses about 0.5 kWh if it draws that power continuously.
How kWh becomes money
Multiply the energy used by your electricity price per kWh. If 0.5 kWh is used and your tariff is €0.30/kWh, the usage cost is €0.15.
Why the label and the meter can disagree
Many appliances cycle or vary their power. A thermostat, compressor, motor controller or computer workload can make average draw lower or higher than a simple rated-power assumption. A plug-in energy meter can capture the accumulated kWh directly.