Watts and kilowatt-hours: power or consumption?

A device's power and the energy it consumes are different quantities. Usage time links them.

Identify power

The watt measures power. A 1,000 W device has a power of 1 kW. That number is not enough to determine its daily consumption: you need to know how long it runs and at what actual power. Two devices with the same power can consume very different amounts of energy if their usage durations differ.

Multiply by duration

A device actually running at 1 kW for two hours consumes 2 kWh. Used that way for thirty days, the theoretical total is 60 kWh. The relationship is energy = power × duration. Using kilowatts and hours gives the result directly in kilowatt-hours. If power is given in watts, first divide it by one thousand.

Estimate variable cost

At a hypothetical tariff of €0.25 per kWh, consuming 60 kWh costs €15. This is the variable cost associated with the entered energy. The standing charge, options and other bill items are not added automatically. Use your own tariff on a consistent tax basis, and distinguish tariff periods if the price changes by time of day.

Understand the limitations of displayed power

Some devices regulate their operation and do not remain at nominal power continuously. For example, a thermostatically controlled heating element alternates between on and off periods. Systematically multiplying maximum power by the entire plugged-in duration may therefore overestimate energy. An actual consumption measurement or the manufacturer's usage information can help you choose assumptions better. The calculator links those assumptions; it does not measure the device.

References

BIPM — International System of Units

Put it into practice

• Electrical appliance cost
• Watts → kilowatts
• Kilowatt-hours → joules

Educational text prepared with AI assistance. The examples use adjustable assumptions and are not personal advice.