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Each bulb earns you £36

Each low-energy light bulb you fit will save £36 over its lifetime.

Assumptions: 60W filament bulb (left) replaced with 20W compact fluorescent (right), run for 10,000 hours (typical life) at 9 p/kWh (typical 'secondary' domestic rate, 2006). The 20W CFL gives more light - it's equivalent to a 75W filament bulb.


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Keep your cool

A room fan may help to keep you cool in summer, but actually it is gradually warming the air. The sensation of coolness is caused by the flow of air across your skin (winter draughts are uncomfortable for the same reason) but heat output from the fan motor, and friction of the blades churning the air, mean that all the electrically energy used by the fan comes out as heat in the room. The moral? Turn off fans in empty spaces. You’ll save power and avoid making the heat worse.

Myth-busting: are full freezers more efficient?

It's sometimes said that a full freezer uses less energy - not true. Research suggest that power consumption hardly varies at all with loading, and simple theory suggests there would be no difference whatever between a full freezer and an identical empty one if the doors were kept closed, and 98-99% of the average freezer's consumption is due to operating in that state.

The problem with a full freezer is that you can't find what you are looking for, so you spend longer with the door open, and that slightly increases energy use. Keeping the freezer full may also obstruct airflow, interfering with thermostat operation and allowing the refigeration compressor to run on when it should cut out.

Much more important in causing high running costs are the following:

  • Having damaged door seals
  • Allowing the condenser coil on the back to get dirty
  • Obstructing air flow over the condenser coil
  • Not cooling produce before putting it in
  • Allowing ice to build up inside
  • Siting the freezer in a warm place
  • Freezing stuff that then gets out of date
  • Opening the door too frequently
  • Keeping the door open too long
The conclusion: store as much or as little as you need, but don't overfill the freezer compartment.

What is the effect of reducing air conditioning temperature?

Got the T-shirt... It is commonly accepted that raising the temperature of your space heating by 1 degree Centigrade adds 8-10% to your fuel costs. There is a similar effect on the running costs of air conditioning if you reduce the space temperature.

An analysis for the UK climate, looking exclusively at the energy used for chilling, shows that the penalty for a one-degree drop in space temperature is about 12% for a building that needs air conditioning all year. For a system that only provides peak comfort cooling in the summer, the penalty is nearly 30% -- and a three-degree reduction doubles its energy requirement.

How many trees would it take to absorb a tonne of CO2?

Carbon offset company say that a tree's ability to absorb carbon dioxide depends on the species, atmospheric temperature, levels of light, water, nutrients, and atmospheric CO2 - and how the woodland is managed. Based on their research they reckon that each sapling they plant will absorb 0.9 tonnes of carbon dioxide per year.

Forests and the European Union Resource Network (FERN) reckon that if this is so, absorbing the UK’s annual greenhouse gas emissions would require planting an area of forest the size of Devon and Cornwall every single year – and maintaining them forever - but they think the concept of accurately predicting trees’ carbon performance is "fantasy" with no solid basis in science.

Not everyone is even convinced that tree-planting works. Critics point out that trees eventually die and rot, releasing carbon back to the atmosphere, and may have serious adverse environmental consequences by soaking up scarce water or decreasing biodiversity.

When does maximising CO2 minimise CO2?

Answer: when you are tuning a burner. The efficiency of a burner (how much useful heat you get out per unit of fuel put in) is largely determined by how much heat is lost in the exhaust gases. Keeping the air:fuel ratio just right prevents the situation where excess air (which is of no use for combustion) just carries away heat and wastes it.

CO2, of course, is one of the main products of combustion, and is produced in proportion to the amount of fuel burned. Excess air dilutes the exhaust, so the percentage of CO2 in the exhaust goes down as the amount of excess air goes up. When the air:fuel ratio is exactly right:

Hence the CO2 emissions are minimised when the percentage of CO2 in the exhaust is maximised.

When is a carbon tax not a carbon tax?

Answer: when it's the UK's Climate Change Levy. This tax is levied on supplies of electricity, gas, coal and liquid petroleum gas to industrial, commercial, and public-sector users. But it's not a true carbon tax because oil (which contains carbon) is exempt, while power from nuclear and large hydroelectric stations (which emit no carbon dioxide) is not.

The omission of oil is odd. Aviation fuel is not taxed at all. Also LPG for road transport is subsdised by a 40 pence per litre reduction in road fuel duty, even though it is far from carbon neutral, while biodiesel (completely carbon-neutral) only gets a 20 pence reduction.

Whatever happened to therms?

If you've been paying gas bills for a while you probably remember paying for 'therms'. A therm is equivalent to 29.3 kilowatt hour (kWh) and the advantage of having gas bills in kWh is that it's easier to compare prices of electricity and gas, which can both be used for heating.

What about a litre of oil? That contains about 11 kWh of energy, depending a bit on what kind of oil it is.

Any form of energy, whether it is used for lighting, heat, or motive power, can be expressed in kWh. For example:

A 100-horsepower engine running for 1 hour   74.6 kWh
1 kilogramme of bottled gas (propane) 13.9 kWh
1 tonne of coal 8,500 kWh (1)
1 tonne of steam condensed to water 630 kWh (2)

1. Depending on type
2. Depending on pressure

And one final obscure fact. Refrigeration power is sometimes stated in 'tons'. One ton of refigeration is equal to 3517 watts.

Who is this promoting fuel efficiency, and when?

"The great waste of fuel must be apparent to the most cursory observer... with regard to the economy of fuel, it has this in particular to commend it, that whatever is saved by the individual is at the same time a positive saving to the whole economy. ...I endeavour to find out how much less fuel the same operation might be performed with, by a more advantageous arrangement of the same fire."

Answer: Benjamin Thomson, Count Rumford, in his essay “Of the Management of Fire and the Economy of Fuel”, March 1798

What does "carbon neutral" mean?"

A carbon-neutral process, building, or vehicle operates on energy sources which are not d erived from fossil fuels. Alternatively, if fossil-derived fuels are used, their carbon emissions can be offset by deliberate activities that permanently absorb an equivalent amount of carbon; the combination can be claimed to be carbon-neutral.

This is rarely quite the whole story because the production of biofuels or uranium (for example) will entail using some fossil fuel. Interestingly, using electricity generated from landfill, mine offgas, and other 'fugitive' methane sources, is better than carbon-neutral because for each molecule of carbon dioxide the power station emits, it removes a molecule of methane which would have been over 20 times as damaging as a greenhouse gas.