Environment Watch

Switch to Energy Saving Lamps and cut CO2 Emissions

The Facts

Only about 5% of the energy used by a normal tungsten filament lamp is converted into light. The other 95% of the energy is dissipated by heat. On the other hand, modern energy saving fluorescent lamps convert about 20% of the used energy into light with the remaining 80% converted to heat.

The following table illustrates the difference between normal tungsten filament lamps and modern energy saving fluorescent lamps.

Name Input Power (Watts) EU Energy Category (A-G) Light Output (Lumens) Expected Lifetime to failure (hours)
Compact Fluorescent 'S' Type 9 A 600 10,000
Homebase Energy Saver Light Bulb 15 A 850 10,000
Philips Brilliant Deco (clear glass) 15 E 110 1,000

How Energy Efficient are Energy Saving Lamps?

On the face of it, the so-called "energy efficient lamps" do not appear to be energy efficient as they only convert about 20% of the energy used to light. Clearly, there still is much that needs to be done by lamp manufacturers to improve the efficiency of lamps.

However, because the function of lamps is to produce light, we should not compare tungsten filament lamps with electronic energy saving lamps of the same electrical power. We need to compare them based on the same light output.

Using an example of a 100 Watt tungsten filament lamp, it gives out just under 5 Watts of light as it is just under 5% efficient. For an electronic energy saving lamp to give out just under 5 Watts of light, it needs to have a rated power of just under 25 Watts as it it around 20% efficient. This is summarised below.

  Tungsten filament lamp electronic energy saving lamp
Light Efficiency about 5% about 20%
Light Output of a 100 W lamp about 5W about 20W
Light Output of a 25 W lamp about 1W about 5W
Light Rating for 5 W light about 100W about 25W

For a detailed analysis of the pros and cons of low energy vs incandescent lamps see the article that was written as a result of an impending ban on incandescent lamps in Australia.

Case Study - How much money do Energy Saving Lamps (lightbulbs) actually save?

This is my actual experience of the failure of two energy saving lamps which actually failed within one week of each other. They were bought in December 2007 and failed in November 2009.

Energy Saving Lamp Costs Tungsten Bulb Costs Notes
Number of days until failure in November 2009 721
Average number of hours lit per day; assuming 6 hours in mid December to 2 hours in mid June 4
Hours to failure 2,884
This is well below the 8,000 hours claimed for these low energy lamps!
£3.10 £1.80 Cost of each lamp in December 2007 or three bulbs.

It is assumed that a tungsten bulb (at 60p each) would last for 1,000 hours before failure. The price of replacements low energy lamps in November 2009 was £3.04 so these calculations are still valid for at least one more year.
+ £3.75 + £17.30 Assumed cost of electricity is 10p per kWh (KiloWatt Hour)

Electricity used for low energy lamp is 37.49 kWh (13 watts (or 0.013 kW)for 2,884 hours)

Electricity used for tungsten bulb 173.04 kWh (60 watts (or 0.06 kW)for 2,884 hours)
= £6.85 = £19.10 Marketting Cost Comparison: These are the costs that low energy lamp manufacturers and sellers tend to quote. That is, the cost of electricity used plus the additional cost of the lamps
The marketting cost comparison implies a saving of £12.25 (£19.10-£6.85) by using low energy lamps. This is not the full picture as some costs have been convieniently ignored. We will not show these other costs.
+ £2.00 + £0.00 Disposal Costs:

Tungsten bulbs are inert and can be displosed of in the weekly kerbside rubbish/garbage collection. They are then sent to landfill.

Energy savng lamps or hazardous waste and must be taken to a disposal site. This may involve a car journey as most amenity sites are not located by a bus stop or in shopping areas. In these cases, the extra cost of car travel needs to be considered and as these are hazardous waste, you may make a special journey to get the lamps out of the house.
= £8.85 = £19.10 Lifetime Comparison: These are the costs for the whole lifetime of lamps and bulbs from purchase, through usage to disposal.
The lifetime cost comparison now implies a saving of £10.25 (£19.10-£8.85) by using low energy lamps. This is still not the full picture for indoor lights. For outdoor lights, this is a good indicator of saving to be made. However, for indoor lights there is another cost factor to be considered. With tungsten bulbs, what happened to all that extra electrical energy that was used? It was converted to heat. Most of the so-called "saved" engery was heat from the tungsten bulb. This would have heated a home in a small way. This heat energy needs to be replaced in the colder months by gas, electric or other heating.
+ £9.31 + £0.00 We have assumed that, when replacing tungsten with low energy lamps:
in summer
no extra energy is needed to replace the heat previously generated by the tungsten bulb
in autumn
½ the tungsten lighting energy "saved" needs to be replaced. That is, 2 hours per day for 6 months
in winter
all tungsten lighting energy "saved" needs to be replaced. That is, 6 hours per day for 3 months.
in spring
¾ the tungsten lighting energy "saved" needs to be replaced. That is, 3 hours per day for 6 months
This gives an average of 11/4 hours per day of the lighting energy saved needs to be replaced. As the average time a lamp is on is 4 hours, the amount of tungsten "wasted" heat energy that needs to be replace is 11/16*wasted heat. The proportion energy supplied to a tungsten bulb that is "wasted" heat is (60-13)/60. So the total cost of tungsten "wasted" heat is tungsten_electricity_cost*11/16*(60-13)/60 = £17.30*0.5385 For simplicity we have assumed that this will be replaced by electric heating.
= £18.16 = £19.10 True cost comparison now shows a saving of £0.94 (£18.16-£19.10) by using low energy lamps. Of course, this figure can change with circumstances; If low energy lamps are left on longer than the tungsten they replaced (which many people do so that they do not have to wait for the lamp to get to full brightness) electricity costs would increase.
  £0.94 Total Saving of a low energy lamp

In summary, not looking at the full picture, low energy lamps appear to save £12.25 (the marketting comparision) but, if the full picture is used, low energy lamps only save £0.94 over their lifetime.

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