What’s the difference between a kW (kilowatt) and kWh (kilowatt hour)?

2nd February 2013 written by Richard Hearne in the category Gas Articles

Ever wondered what the difference is between a kilowatt and a kilowatt hour? You’re not alone. We’ll start by making it really simple and look at the components of the words themselves. Just like a kilogram is 1000 grams, the ‘kilo’ in kilowatt stands for 1000, so there are 1000 watts in a kilowatt. The ‘watt’ is the unit of power named after the Scottish inventor and scientist James Watt.

James Watt (kilowatt)Watt, born in Greenock, Scotland was born in 1736 and died in 1819 . He was a key part of the start of the industrial revolution in the United Kingdom. He famously worked on producing steam engines which were much more efficient. Watt is now perhaps more famously known for his association with watts and kilowatts to describe energy and power.

The Kilowatt (kW)A Kilowatt is a measure of power and used to describe the power which is being generated or used at any given moment. Sometimes kilowatts may be described as a ‘load, instantaneous demand, peak consumption, or peak demand’. A kilowatt is another way of measuring the flow of power over time.  One kilowatt is equal to a thousand watts. One watt is equal to one joule per second. A typical household condensing boiler may use around 18 kilowatts and might appear on the appliance or in the user manual as an ‘input rating’. A plumber or Gas Safe engineer can help if necessary.

The Kilowatt Hour (kWh)Kilowatt hours measure energy used over a period of time. To calculate a kilowatt hour, simply multiply the usage of power in kilowatts by the number of hours. A typical takeaway restaurant may use 100,000 kWh of energy over a year. This is also known as an AQ or Annual Quantity by gas suppliers and may be shown on gas bills, especially for larger non domestic customers.



A boiler using 20 kilowatts over two hours will consume 40 kWh. Over a 30 minute period, the figure would be 10 kWh. To calculate the power required to create an amount of energy used, divide power by time. So, 40 kWh of energy divided by 4 hours would result in 10 kilowatts of power having been used.

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