Energy and CO₂ Emissions from Common Fuels

The energy output and emissions of fuels such as gas, oil, coal and wood vary. Furthermore, part of the energy from burning such fuels appears in the form of steam.

In domestic applications this part used to be lost, but condensing boilers can recover some of this energy – these are now required for all new installations and replacements.

For transport it is not easy to evaluate how much of this energy is used, and therefore some references give the energy content of petrol and diesel as roughly 10% less than we state here.

The figures in the following table should therefore only be assumed to be accurate to about 10% in most cases.

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Force, Work, Power, Energy and Heat Units

Force

newton (N)

The force required to accelerate 1 kilogram at a rate of 1 metre per second per second. The force of gravity at the earth's surface is 9.8 newtons per kilogram.

Work

joule (J)

The energy expended when a force of 1 newton moves through 1 metre.

Power

Power is the rate of doing work, and is used as the rating of an electrical appliance, or even a power station. For example, a 60 watt light bulb, a 2 kilowatt electric fire, a 3 megawatt wind turbine, or a 2 gigawatt power station.

watt (W)

A rate of work (i.e. energy expenditure) of 1 joule per second.

kilowatt (kW)

1000 watts.

megawatt (MW)

1000 kilowatts, or one million watts.

gigawatt (GW)

1000 megawatts, or one billion watts.

Energy

joule (J)

The unit of energy is also the joule. When work is done energy is transformed from one form to another.

kilowatt-hour (kWh)

The energy used when a device with a power rating of 1 kilowatt is used for 1 hour, or equivalent. It is equal to 3,600,000 joules, 3413 BTU or 860,000 calories.

household

The energy of wind farms or other energy sources is often given in 'households'. For this purpose the electrical energy used by a typical household is assumed to be about 4700 kilowatt-hours per year – if spread evenly over the year this would be a continuous load of 0.54 kilowatts. Thus a wind farm with a peak rating of 3 megawatts, which is expected to provide an average output of 30% of peak, or 900 kilowatts, may be said to be able to supply 900/0.54 = 1666 households. This only supplies the typical electrical energy load of a household. The typical total energy consumption of a household, including heating by fuel such as gas, is closer to 20,000 kilowatt-hours per year.

Heat

calorie (cal)

The energy required to heat 1 gram of water by 1 degree Celsius at 15 degrees Celsius and standard atmospheric pressure. A calorie is equal to 4.187 joules. The energy in food is measured in 'Calories', which are actually 1000 calories.

U-value

The heat conductance of a material. U = 1 means energy is transferred at a rate of 1 watt (1 joule per second) through 1 square metre of material with a temperature difference of 1 degree Celsius across it. This is equivalent to 3600 joules or 860 calories per square metre per hour.

gas unit

The unit of gas supply is 100 cubic feet. In the UK, because of the slight pressurisation of the supply, this is assumed to be 102.264 cubic feet at standard temperature and pressure. The energy in mains gas varies through small differences in composition: in the Blewbury area it is billed at 39.3 million joules per cubic metre, and one unit is assumed to supply 31.59 kWh.

BTU (British Thermal Unit) – No longer used in the UK

The energy required to heat 1 pound of water through 1 degree Fahrenheit, equivalent to 252 calories or 1055 joules.

therm – No longer used in the UK

100,000 BTU, 105,500 joules, or 29.3 kWh. 1 unit of gas generates approximately 1 therm of heat.

Miscellaneous

US gallon

0.834 imperial gallon, or 3.79 litres. (US pint is 16 fluid ounces not 20, and the US ounce is slightly bigger than the imperial ounce.)

barrel of oil

42 US gallons, 159 litres, or 0.1364 tonne.

horsepower

0.746 kilowatts.

oil energy equivalents

1 barrel supplies about 6 billion joules; 1 tonne supplies about 42.6 billion joules.

tons and tonnes

1 metric ton, often written tonne to avoid ambiguity, is 1000 kg, or 2205 pounds.
1 imperial ton is 1016 kg, or 2240 pounds (sometimes called 'long ton').
1 US ton is 907 kg, or 2000 pounds (sometimes called 'short ton').

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Typical U-Values

Because of the range of materials and construction used, the actual U-values for each kind of material vary significantly. The following are only indicative.

Note: Metric units are used in the UK. The USA still uses imperial units, which can cause confusion. For example the value of U – the measure of heat conductance of a material – is 5.6 times greater than on American websites, as they are using the imperial unit of 1 BTU per square foot per degree Fahrenheit, while most other sites are using the metric unit of 1 watt per square metre per degree Celsius. Thus a heat conductance of 0.56 U on a UK site will be the same as a heat conductance of 0.1 U on an American site.

The UK's Standard Assessment Procedure defines the following U-values to be applied for new dwellings when the actual materials are not known:

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