A Beginner’s Guide to Energy and Power
Article submitted by N Packer, Staffordshire University, UK, February 2011.
Energy
Energy is the ability to do work.
As is often the case, the units of energy are named after a researcher whose contribution to a
particular field is central to its understanding. The internationally (non-US) recognised unit of
energy is the Joule.
Physicist & brewer James Joule (1818-1889) spent a career (35 years) examining
various forms of energy conversion with much greater accuracy than researchers
in the field of study had before. He looked at many cases of the conversion of
mechanical energy to heat or thermal energy. He found equivalence between
the expenditure of an amount of work and the resulting heat. His work
confirmed the principle of energy conservation to be exact when friction
and air resistance is taken into account.
1 Joule is not exactly a large amount of energy and so another unit, the kilowatt-hour (abbreviated
to kWh) is more common.
The conversion is: 1 kWh = 3.6 million Joules (abbreviated to 3.6MJ).
In terms of continuous operation time 1 kWh of energy will, for example, give you:

2 days use of a small compact fluorescent lamp

10 hrs use of television

5 hrs of PC time

70 minutes of microwave oven use

60 minutes of lawn mowing

4 minutes of domestic boiler at continuous full power
 100 seconds of a small car at continuous full power
You can see that the kWh itself is not a particularly large amount of energy and the term Megawatthour (abbreviated to MWh) i.e.1000kWh, is also in use.
Fuel comparisons
Historically we have largely acquired our energy from the consumption of fossil fuels so considering
and comparing fuel sources in terms of the amount of energy they contain per unit of purchased
source would be instructive. Some guidance (using average values) is given below.
1 kg of anthracite (4% moisture content) = 36MJ
= 10kWh
1 kg coal (5-10% moisture content)
= 37MJ
= 10.3kWh
1m3 of natural gas
= 39MJ
= 10.8kWh
1 litre of gasoline
= 34MJ
= 9.4kWh
1 litre of diesel fuel
= 40MJ
= 11.1kWh
1 litre of gas oil
= 41MJ
= 11.4kWh
1 litre of fuel oil
= 44MJ
= 12.2kWh
(The above values are based on Gross Calorific Value i.e. assumes water formed and liberated during
combustion is in the liquid form.)
For comparison a 1kg of a renewable fuel like a woody biomass might typically contain 4.2kWh.
Energy content pricing is an interesting concept. For example a litre of diesel fuel contains
approximately 18% more energy than 1 litre of gasoline and yet its price at the fuel pump is only a
few percent in excess of gasoline.
Power
Power is the rate of doing work.
Again, the units of power are named after a researcher whose contribution to a particular field is
central to its understanding. The internationally (non-US) recognised unit of energy is the Watt.
Scottish engineer and inventor James Watt (1736-1819) was employed
by Glasgow University to repair a model of the most advanced steam
engine of its time, the Newcomen engine. Watt devised a range of thermal
and mechanical improvements to the device that improved its efficiency
by 300% allowing it to become universally adopted wherever mechanical
power was required.
A Watt is an expenditure of 1 Joule per second. The value of the power rating is commonly stated
on energy consuming and energy generating devices and tells us something about how quickly
energy is being either used or made.
1 Watt is not exactly a large amount of power and so a multiple unit, the kilowatt (abbreviated to
kW), i.e. 1000Watts, is commonly in use.
For illustration purposes the power rating of some devices is given below:








Personal computers: 50 – 200W
Microwave ovens: 650-850W
Lawn mowers: 1-1.7kW
20m2 of mono-crystalline photovoltaic: 2.5kWpeak
A domestic biomass pellet boiler: 15kW
A 50m diameter horizontal axis wind turbine : 500kWpeak
Steam turbines: up to 60MW
Gas turbines: up to 100MW
(consumption)
(consumption)
(consumption)
(generation)
(generation)
(generation)
(generation)
(generation)
Energy – power relationship
Energy and power are inextricably intertwined. The application of power for a given time will either
result in (generation) or require (consumption) energy.
Mathematically the link is simple if you remember to get the units (kWh, kW and hrs) right.
The link is:
Energy (kWh) = Power (kW) x time (hrs)
Examples
1.
A electric convector heater rated at 1kW left on for:

1 hr will consume

30 minutes will consume 1 x (30/60) = ½ kWh
2.
A compact fluorescent lamp rated at 20Watts left on for
8hrs will consume (20/1000) x 8 = 0.16kWh
1 x 1 = 1kWh
Final words
Power (measured in kilowatts) and energy (measured in kilowatt-hours or Joules) are not identical or
interchangeable. However their conversion is relatively simple with a little practice.
If you would like to learn more, follow the links below.
http://www.kayelaby.npl.co.uk/
http://www.simetric.co.uk
Neil Packer is a Chartered engineer and Senior lecturer at the Faculty of Computing, Engineering and
Technology, Staffordshire University, UK. He has been teaching thermo-fluid and environmental engineering
for nearly 20 years and acts as a Low Carbon Consultant providing a range of energy services to business,
industry and local authorities.
Contact details:
Faculty of Computing, Engineering and Technology
Staffordshire University
Beaconside, Stafford, ST18 0AD
Tel 01785 353243
email n.packer@staffs.ac.uk
This information was presented as part of the Renewable Energies Transfer System Project (RETS) funded by
INTERREG IVC through the European Regional Development Fund. The project time line is January 2010 to
December 2012. For more information and to take part in our online community visit: http://www.retscommunity.eu/