Guide

Electricity consumption and power at home

Power in watts and energy in kilowatt-hours are constantly confused. Power says how fast energy is converted, the kilowatt-hour how much energy flowed in total.

Power and energy are two different quantities

Power is measured in watts and always describes a rate, even when stated as a continuous value. Energy is measured in joules or kilowatt-hours and describes an amount. One kilowatt-hour is the energy a device of 1000 W converts in one hour: 1 kWh = 3,600,000 J.

The calculation for annual consumption

Consumption is power times time. A device drawing 100 W continuously uses 2.4 kWh in 24 hours and about 876 kWh in a year. So if only the power is known, consumption can be calculated without measuring — provided the device really runs continuously.

Watts, kilowatts and the confusion with watt-hours

One kilowatt is 1000 watts, one megawatt 1,000,000 watts. A kilowatt is a power, a kilowatt-hour an energy — the difference is the time unit. A device of 1 kW running for one hour uses 1 kWh. The same device uses 0.5 kWh in 30 minutes but still has 1 kW of power.

Calories, joules and BTU as further energy units

Energy appears in several units: joules in the SI, kilowatt-hours in electricity and on bills, kilocalories for food and BTU in heating engineering. All of them describe the same quantity. 1 kWh is 3.6 megajoules or about 860 kilocalories, and 1 kcal is 4.184 kilojoules.

Matching conversions

Matching tables

Unit family: Energy

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