Energy Definitions - Explained
A kilowatt hour or kWh is probably the easiest energy definition to explain to the layman.
If you look at an electricity bill you should see the kWh unit. As of January 2007, in the Republic of Ireland, the ESB are charging a domestic rate of 14.35c for every kWh that is used.
Your electricity bill will read something like;
The kWh or kilowatt hour on your bill is the amount of kilowatts your house uses in an hour. All modern electrical devices state their wattage. A typical kettle for example is 2400W or 2.4 kW (one of the highest household appliance consumers of power). So if you had an electric kettle operating for one hour, it would cost you 2.4kW X 14.35c X 1hr. Therefore your average kettle costs 34.44c per hour to operate. This may not hit your pocket but for electric showers, the efficiency of generating steam from hot water to make electricity to send to people’s houses over miles of electricity line to reach their electric shower to heat water to make steam does raise some questions.
Why is a typical electric light bulb not very efficient? The reason is because so little of the electrical energy is converted into light energy. A lot of energy is used up through the creation of heat energy, as anyone that has grabbed a hot bulb will know. A watt is the official unit of power and named after James Watt. Power is defined as the rate at which energy is converted from one form to another. Therefore the Watt is not only used for electrical items but also for thermal usage. Officially a small “e” or “th” should be placed after the “W” to show if the user is talking about electrical or thermal power. A typical electric light bulb rated at 100W converts electrical energy at a rate of 100 joules per second into light and heat.
The Joule is the scientific measure of energy (named after James Joule). Energy is defined as the ability to do work or the ability of a body to move against a resisting force. A joule is a very small quantity of energy. This is obvious if you look at the “energy per bar” rating on the back of a chocolate bar wrapper. There are usually around 1000 kJ or one million joules in many bars. Yet if somebody had to do a day of heavy, manual labour, they would not eat only chocolate all day. The average male uses 2,500kcal per day, the average female uses 1,500 - 2000 kcal. The scientific definition of a calorie is the quantity of heat required to raise the temperature of 1 gram of water by 1ºC.
A calorie is equal to 4.184 joules. In the Republic of Ireland, we use approximately 3,000,000,000,000,000,000 calories of energy in a year. The TPER (Total Primary Energy Requirement) of the Republic of Ireland in 2002 was 620 PJ (petajoules) or 620 X 1015.
The typical energy content of some fuels are given in the table below.
Conversions of common energy definitions are given below;
1 joule = 0.239 calorie
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1 TOE (“tonne of oil equivalent”) = 1 X 109 joules
1 BTU = 1,055 joules
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1HP = 746 joules per second.
1 BTU per hour = 0.29W