The Kilowatt (kW) and the Kilowatt Hour (kWh) as Units of Energy
The Kilowatt (kW) and the Kilowatt Hour (kWh) as Units of Energy

The kilowatt (kW) is a unit of power, not energy. It quantifies the rate at which energy is transferred or converted. One kilowatt equals 1000 watts.

A watt is defined as one joule per second. So a kilowatt is effectively 1000 joules transferred or converted every second.

The term kilowatt is commonly used to express the power output of engines and the power consumption of tools and machines. For example, if a light bulb has a power rate of 100 watts, it is converting energy at a rate of 100 joules per second.

A kilowatt hour (kWh), on the other hand, is a unit of energy. It is used to measure the amount of energy that’s been used over time. As its name implies, it represents one kilowatt of power expended for one hour.

When we say an electrical device uses 1 kWh of energy, that means it has been running at a power rate of 1 kW for a period of one hour.

The energy used by appliances is often given in kWh because it’s a convenient size for expressing the large amounts of energy typical in electrical consumption. This is the unit of measure you will likely see on your energy bill.

Remember to be able to convert between different units of energy. For example, 1 kWh is equivalent to 3.6 million joules (J).

Reallife application: If an electrical device has a power rating of 2 kW and it’s switched on for three and a half hours, the energy used can be calculated by multiplying the power (in kW) by the time (in hours). So energy used = 2kW x 3.5h = 7kWh.

Unit understanding and conversion is an essential part of problemsolving related to domestic electricity. Ensure you are comfortable with these terms, and practise using them in relevant calculations.