Difference between revisions of "CFL"

From DIYWiki
Jump to navigation Jump to search
(→‎Kids' room lamps: replace hand waving accident verbage with actual accident stats)
Line 431: Line 431:
* Claimed equivalence to 60W is not actually too far off with these.  
* Claimed equivalence to 60W is not actually too far off with these.  
* 9/10
* 9/10
=== Tesco Stick Lamps===
I bought some in about Feb 2008 (priced 10p). By Nov 2010:
* Big fat 3-stick one in the lounge was still in use last month, with
fairly heavy use.
* Short 3-stick ones still in use and fine in the bedroom, with moderate
* SES reflector spotlights in the kitchen started looking a bit dim,
moderate use but switched on and off a lot.

Revision as of 17:11, 4 November 2010

  • Compact Fluorescent Lamps (CFLs) save power consumption and carbon emissions compared to Filament lamp.
  • Cost saved is normally many times their purchase cost (and this is with taking into account the value of the heating effect of filament lamps).
  • Energy saved is many times bulb manufacture energy
  • CFLs vary in light quality, from excellent to awful.
  • CFL lighting does not save a lot compared to other ways to save energy & money, but its usually an easy option that saves quickly.
  • Some people don't like the look of CFLs
  • The energy and cost savings are not big.

Power Equivalence

Packet quoted power equivalents are misleading if compared to standard GLS filament bulbs. This continuance of misleading claims has put a good many people off using these light sources, as fitting a lower output lamp often makes a room look badly lit. The packet figures are actually comparisons to softone filament lamps, which are less efficient than standard GLS filament lamps.

CFLs are at best 4x as energy efficient, so 1/4 the filament wattage is a good first approximation. (The ratio changes for lowest power lamps because 240v filament lamps fall off in efficacy at low powers more than CFLs do.)

Approx power equivalence:

  • 100w: 25w
  • 75w: 20w
  • 60w: 15w
  • 40w: 11w
  • 25w: 5w
  • 15w: 3w

Power equivalence of reflector lamps

Reflector versions of CFLs and CFLs fitted in reflector fittings fare worse, with a lower percentage of the light going in the wanted direction than with filament lamps, and the large area of lamp tubing absorbing some reflected light. The result is that power equivalence is closer to 3:1 for reflector uses.

Miniature compact reflector lamps can see power equivalence nearer 2 or 2.5x, as much of the light emitted from the rear of the lamp is lost.

Sample Saving Calculation

To see if these lamps really save anything, we will compare a 100w filament bulb with a 25w CFL used in a centrally heated house.

We will use the following figures, which will vary from case to case:

  • Light use: 5.5hrs/day average = 2000 hrs/year
  • Electricity cost: 10p/unit
  • Gas cost: 3p/unit
  • boiler efficiency: 83%
  • Central Heating is used: 8 months per year
  • CFL cost: £2.50
  • CFL lifetime: 8000 hours
  • filament lamp cost: 20p
  • filament lamp average life: 1000 hrs.
  • Electricity generating plant efficiency: 40%

Money Savings

Annual cost of use of one filament light:

  • 2x lamps @ 20p each = 40p for lightbulbs
  • 2000 hrs x 100w = 200kWh
  • 200kWh x 10p/unit = £20.00 for electricity

Total cost of operating one filament light fitting = £20:40 per annum.

Annual cost of use of one CFL light:

  • 2000 hrs / 8000 hrs x £2.50 = 62p for lightbulb
  • 2000 hrs x 25w = 50kWh
  • 50kWh x 10p = £5.00 for electricity

Total cost of operating one filament light fitting = £5:62 per annum.

The CFL produces less heat (25w of heat instead of 100w), so this extra heating effect of the filament lamp must be made up for by the gas for 8 months of the year.

  • 100w - 25w = 75w
  • 75w x 2000 hrs x 8/12 months = 100kWh.
  • At 83% efficiency, 100kWh output requires 100/.83 = 120kWh gas use.
  • Cost = 120kWh x 3p = £3.60
  • Total cost of cfl use = £5.62 + £3.60 = £9.22 per annum.
  • Saving = £20.40 - £9.22 = £10.78

Saving = £10.78 per year per light fitting.

A household using 8 such fittings would therefore save in the region of £86 per annum.

Savings are higher in unheated locations.

With air conditioning

When a/c is used the savings for CFLs are greater.

Example: using the example above, plus:

  • AC use: 2 months per year
  • AC Energy extract/use ratio: 3:1
  1. 2 months CFL use uses 2/12 x 2000hrs x 0.025kW = 8.3kWh
  2. 2 months Filament use uses 2/12 x 2000hrs x 0.1kW = 33.3kWh
  3. Filament lamp thus uses an extra 25kWh over 2 months.
  4. This requires ac electricity use of 25/3 = 8.3kWh to extract it
  5. 8.3kWh @10p/kWh costs 83p

Thus each light fitting using a filament lamp costs another 83p in electricity to extract it heat. For single bulb in centre of room this isn't much, but for multiple downlighter spot halogen lighting the amount adds up quickly. And this figure is on top of the savings mentioned above.

There is a second extra cost associated with filament lamps and ac as well. If you're using rooms full of halogen filament lamps, you need a bigger ac unit to get the same target temperature, meaning a higher purchase price. Or you use the same unit, run it for more hours and sit in a hotter room.

Offgrid use

Electricity for offgrid use is particularly expensive. It can cost as much as 50p per kWh. This makes the cost of running filament lamps far higher than CFLs. Linear fluorescent is popular off grid as it has even better energy figures than CFL.

Energy & carbon Savings

When not to use CFL

CFLs are not well suited to all applications.

Outdoor PIR lights

PIR lights switch the lamp frequently and for short periods. This will cause premature failure of CFLs.

Cold outdoor weather will mean reduced light output and prolonged warm up periods, which will give poor results on a PIR light.

Bathroom Lights

If a bathroom light is switched on frequently for very short periods, the CFL will fail early.

To replace linear fluorescent lights

CFLs work fine in place of linear fluorescent, but are not as energy efficient as them, so replacement will not yield any further money or energy savings. Linear fluorescent are the most energy efficient type of lighting normally used in homes.

Some types of timer

A minority of timers pass neutral current through the lamp when off. With CFLs this will cause a regular momentary flash when off. Timers with a neutral connection do not cause this issue. See below for resolving this.

On very long switched wire runs

A CFL may flash occasionally when off if the switch wire run is especially long. See below for resolving this.


Poeple with photosensitivity can react badly to CFL light. Photosensitivty is caused by medical conditions such as Lupus and some medications. CFLS with an outer glass envelope over the tube don't cause this.

Occasional flashing

In exceptional cases a CFL will flash occasionally when switched off. This is due to wiring capacitance passing a tiny current, which gradually charges the CFL's reservoir capacitor, and after a while it attmpts to start, giving a momentary flicker.

2 conditions tend to cause this:

  • an especially long switch wire run
  • supply switched on the neutral instead of live pole

Other than moving a neutral switch to the live line, possible solutions include:

Filament Lamp

Appliance lamp 4218-2.jpg

If a filament lamp is on the the same switch (or PIR/ timer) as the flickering CFL, the problem will not occur. The filament lamp can be a low power type, such as 15w.


One resistor 0982-12.jpg

It is also possible to stop the problem by fitting a suitable resistor in parallel with the bulbholder. Use either:

  • A 270k to 470k 400v rated resistor, or
  • 2x 150k to 220k 200v rated resistors in series, with both the same resistance value.

Resistor choice explained:

  • If using the common 200v 0.3w type resistors, they're only 200v rated, so 2 in series must be used, and of the same value so the voltage drop across each is equal.
  • A 270k resistor will dissipate 0.2w on 240v mains.


Capacitor 2756-4.jpg

A capacitor can be used in the same way as a resistor. These should not be fitted inside the lampholder itself for temperature reasons. Capacitors consume no power.

A 10nF 275vac capacitor with X2 dielectric is suitable.

Note that capacitors marked as 250v rated are totally unsuitable for mains use. Before the advent of X & Y capacitor classifications, 600vdc caps were often used on unfiltered mains, but this is no longer compliant.

2 way switch

Many light switches are 2 way switches.

Live  ----o---
              o------ switched live line to lamp

Conventional use

                                    o---/\/\--- Neutral
switched live line to lamp  ----o---
                                    o------ Live

No-flash variation

R1 is a single 270k resistor rated to 400v, or 2 identical 150k 1/4w resistors in series.

CFLs have other advantages

As well as saving money and energy compared to filament lamps, CFLs have characteristics that make them particularly well suited to some tasks.

Bedside lamp

Picking a CFL that comes on at reduced brightness initially, as a minority do, is a lot more comfortable on the eye than the instant full output of filament lamps.

Kids' room lamps

CFLs represent a lower fire risk than hot filament lamps. A knocked over plug-in light with a CFL is not a fire risk, and a CFL with something draped over it is very much less likely to cause a fire than a filament lamp.

Kids are not the most responsible and careful members of society, and their behaviour around plug-in lights is no exception.

Data supplied by the Department of Communities and Local Government, 11/02/10, suggest that in 2007 portable lighting appliances were responsible for 944 fires, of which 155 caused injury, and 4 resulted in a fatality.

Hot weather

With a/c

During air conditioning use, you pay twice for the extra power used by filament lamps. First the filament bulb uses more electricity, then the a/c uses power to extract that heat from the building.

Without a/c

When no a/c is in use, the heat of filament bulbs just goes to increase the indoor temperature further. How much further depends on the lighting setup, and varies. Temp gains can be as high as several C in some cases, particularly with halogen lighting. A few degrees C has a fairly big effect on comfort.

Inspection lights

Small CFLs are generally more robust than filament lamps, though not by any huge margin. This may result in less bulb failures. If your use is rough enough to break either type of bulb regularly, LEDs would generally be better.

Night lights

Long on-times suit CFLs well. Much lower run temperature makes CFL less of a fire risk than filament lamps, which is especially useful for plug in lights used at night.

CFLs are readily available down to 3w, which cost very little to run (around £2.50/year for all night use every day).

Backup lighting

Long life and low power consumption make CFLs well suited to back up other lighting in areas where light failure would cause a problem. Inaccessible stair lighting is one such example, whether the main lighting is CFL, filament, or any other type.

See facilities lamp and electrodeless lamp sections

Special Purpose Lamps

High power Lamps

CFLs of powers upto over 100 watts are available.

Electrodeless Lamps

Electrodeless CFL Lamps have much longer lifetimes than standard CFLs, with life expectancies of 15,000hrs and up.

The Genura is an electrodeless R80 lamp with at least 15,000 hour life expectancy, and a £20 price tag to match. Even at these prices, total cost of ownership is still lower than filament lighting by a good margin.

Lamps with even longer life cost more.

Facilities Lamps

Extra long life lamps are available with lower price tags than the Genura, but in general you will pay a bit extra to avoid changing lamps as often. Life expectancies vary, but 12,000 or 15,000 hours is fairly typical. These are generally called facilities lamps, and are usually used in locations where there is a cost to relamping.

For domestic use these lamps are useful for locations where relamping is inconvenient. Examples include:

  • Lights mounted at a great height.
  • Shared entrance area lights
  • Lights where the occupant is unable to relamp, eg disabled or elderly.
  • Exterior light fittings that are not quick & easy to open (but not PIR lights).

Yellow CFLs

Yellow CFLs can be used outdoors to reduce attraction of insects. Yellow CFLs have an even greater energy efficiency advantage over yellow filament lamps.

Titanium Dioxide CFLs

Titanium Dioxide coated CFLs cause chemical reactions in the air touching the TiO2 coating. Water vapour is turned into hydroxyl radicals, which oxidise odours, and kill bacteria, viruses, & mould spores.

Blacklight CFLs

Black light bulb2.jpg

Blacklight CFLs are available for quite modest prices, and are an easy way to add a little style to parties. Also used to check bank notes and detect biological residues.

Never use germicidal UV lamps for this, these cause eye damage.

Colour CFLs

CFLs are available in many colours besides white, such as red, pink, orange, yellow, green & blue. While such lights are of limited use, the ratio of their energy efficiency compared to coloured filament bulbs is very much greater than 4:1, for reasons it is not necessary to delve into here.

Colour CFLs may be used for decorations, parties, etc.

A colour CFL fitted temporarily in the porch also makes it very easy for visitors to find your house - though red is best avoided for most of us.



CFLs continue to be less popular than filament bulbs despite the benefits. Reasons most often cited for this include:

  • not liking the light
  • lamp didn't fit in the light fitting

Other reasons include:

  • not liking the appearance of the bulb itself
  • can't be bothered with them
  • contribution to energy and pollution reduction too small to affect climate outcome.
  • money saving too small to care
  • appearance more important than savings
  • can't dim CFLs easily
  • higher level of premature failure in some applications
  • CFLs dont reach full brightness instantly
  • light output level drops more through its life than with filament lamps
  • safety concerns regarding mercury content
  • stated power equivalence on package untrue (true, but picking a genuinely equivalent lamp is hardly a challenge)

There are also some reasons sometimes cited that are either not really correct or not entirely logical:

  • CFLs can take a long time to reach full brightness when they are cold (this was an issue with early types, but few CFLs behave like this today.) This still happens where unsuitable types are used outdoors)
  • safety concerns regarding fire hazards if the ballast shorts out (ballasts contain a failsafe safety device for this. Total fire hazard is less than that of filament bulbs)
  • higher bulb cost (while true, the energy savings are greater than the added bulb cost)
  • not knowing if they really save energy (they do)
  • not knowing if they really save money (they do)

Medical conditions

Those people who show photosensitivity, notably those with Lupus, may have a severe skin reaction to UVA light emitted by some CFLs. This applies to people who need to avoid daylight, which also contains UVA. CFLs with an outer glass cover over the tube don't cause this problem.

Faulty CFLs can trigger epileptic seizures in epileptics intolerant to strobing. However

  1. such a failure mode is unusual
  2. a dying lamp that was flashing would normally be removed from service immediately by any homeowner.


CFLs do contain a trace of mercury, which is released on disposal or breakage if the lamp is not recycled. Burning coal to produce the extra power used by filament lamps might release more mercury than the mercury contained in a CFL lamp, but the mercury released by burning coal is not concentrated in one's living space.

Filament lamps contain thorium, another toxic heavy metal.


Mandated CFLs

Many people object to the government mandating the fitting of CFL-only light fittings in new builds, and these fittings are in most cases promptly removed. The only thing left is a sour taste, and resentment at the waste of the exercise.

Some support this policy on the grounds that at least some energy efficient fittings are left in service.


Bulb Choice

Available lamp models change, so try one each of a few different ones to see which gives best results.


Like linear fluorescents, CFLs come in more than one type of white. 2700K or warm white are equivalent to filament lamps, daylight lamps give a very much colder light not liked by most people.

Colour temperture may be indicated in any of 3 ways:

  1. Kelvin figure. 2700K - 3000K is warm, 4500K is cool, 6800K is ice cold.
  2. Kelvin temp and CRI may be encoded into one figure. 827 means 2700K with a CRI in the 80s%, 854 means 5400K.
  3. Colour names such as warm white or daylight. Daylights give a very cold icy light, and are rarely appreciated.

If you have an unmarked lamp, the simplest way to check is to put 2 bulbs side by side, a filament bulb and the cfl, to see if they give the same light colour. Some give an accurate match, some are slightly pinkish, and some give a much colder light. Most people prefer bulbs that match filament lamps.

Outdoor Use

CFLs take time to warm up in winter outdoors. Outdoor rated bulbs are available. These often have glass or polycarbonate outer covers.


For R80 fittings where the bulb is not visible, ie is wrapped around by the fitting cowl, try using miniature non-R80 bulbs. GE's microspirals fit well. For some reason R80s are either steeply priced (Genura at around £20 each) or not as attractive as filament R80s. Despite the price tag, even the long lived Genuras save money in the end.

20w Bulb Orientation

CFLs of 20w and more are designed to be used base down. They can be used base up too, eg in ceiling pendant fittings, but some types will have shorter lives than rated in this position. This does not prevent them saving money and energy though, as the energy saving is far greater than the bulb cost.

Enclosed Fittings

Most CFLs are happy enough in enclosed fittings, but a minority aren't. Sometimes light output falls, or the bulb fails early.

Fitting Power Ratings

Generally you don't need to worry about fitting power ratings when using CFLs, as the CFL power rating will be far below the fitting's limit. But with enclosed fittings or fitings where you need to squeeze maximum light output, this is best considered to avoid bulb failures.

A 25w rated bulb holder can cope with anything upto 25w of lightbulb, regardless of what technology that bulb uses.

But a 25w CFL would get too hot in a 25w max rated fitting, because CFLs are designed to run cooler than filament bulbs. So the practical recommended limit for CFLs is below the light fitting rated max. Its impossible to provide an exact number, as different CFLs will tolerate differing heat levels.

If your fitting is not giving enough light with a filament bulb of max rated power, using a CFL of half the power will provide a good deal more light.

If too high a power CFL is used, the bulb will run hot and fail early. However this is not the fire risk that too high powered filament bulbs present, its only a risk to the lifetime of the bulb.

Some bulbs may also produce reduced light output when run too hot.

User Reviews


  • Light quality excellent
  • bulbs reliable
  • 20w+ bulbs achieve full rated life when operated cap up
  • recommended
  • 10/10


  • Light quality good
  • Candle bulb glass covers break easily
  • 8/10


  • Light quality so-so
  • bulbs reliable
  • Good power/size ratio with mini spirals
  • 7/10


  • Bulb life unimpressive
  • 5/10


  • Light quality horrendous
  • Light output started a bit low, and declined badly
  • 1/10

Screwfix R80s

  • Lower light output than 60w R80 filament bulbs
  • Light quality so-so
  • Slightly longer bulb looks bad in fittings where the bulb is visible.
  • High price
  • 4/10

Megaman ultra compact candle

These are designed to act as a replacement for candle effect bulbs, and are very similar in shape and overall size.

  • 7W bulb gives similar light output to a 40W filament when looking at the bulb, although a slightly reduced output into the room.
  • Light quality is a fairly good match in both brightness and colour
  • Warm up is reasonably quick
  • Price approx £6 / each
  • 8/10

GE Spirals

  • High efficiency due to 8mm tube
  • Good light quality from the 2700K ones
  • Unpleasant light from daylight ones
  • Short length makes them usable in R80 light fittings where the wider beam is acceptable
  • 15w bright enough to replace 60w filaments
  • £2.28 at Tesco and half price offers frequent - but check you get 2700K
  • long life & no premature failure problems, despite operating cap up
  • 9/10

Tesco large candle

  • Probably made by GE - but not all boxes carry a brand
  • Slightly green light quality
  • Slowish warm up
  • Colour match between bulbs seems poor - identical bulbs bought at the same time and used in the same multi lamp fitting have never looked the same.
  • Longevity doubtful - first failure in about 1.5 lifespans of a filament lamp operating cap down.
  • 3/10

Tesco 11W Stick Lamp

11W Stick lamp - click to enlarge
  • better than normal output toward the red end of the spectrum, giving better colour rendition than most CFLs
  • Warm up is not too slow, and the initial light output is usable.
  • Colour accuracy between bulbs is good.
  • Claimed equivalence to 60W is not actually too far off with these.
  • 9/10

Tesco Stick Lamps

I bought some in about Feb 2008 (priced 10p). By Nov 2010:

  • Big fat 3-stick one in the lounge was still in use last month, with

fairly heavy use.

  • Short 3-stick ones still in use and fine in the bedroom, with moderate


  • SES reflector spotlights in the kitchen started looking a bit dim,

moderate use but switched on and off a lot.


For a list of suppliers, see Suppliers Article.

See Also

Dollar store CFL review, many problems