Selecting replacement lamps.


Following numerous conversations with customers about their problems selecting replacement lamps, this page has been produced to help with understanding why choosing new light fittings, or selecting lamps for existing light fittings can be confusing, since the introduction of energy saving lamps over recent years. 

Recap: Back in the good-old-days, typical houses had one pendant lamp holder in each room, a fluorescent strip light in the garage (& maybe kitchen), and a glass globe enclosed fitting in the bathroom. Most light bulbs were GLS filament lamps and we tended to select a new lamp based on power rating, appearance, shape & fitting type, such as;  (25, 40, 60, 100, 150watt) / (Pearl or clear glass) / (Round or candle shape) / (Bayonet or screw thread fitting)  Buying replacement lamps was simple. Even as progression brought us spot lamps, automatic security lamps, dimmer switches etc, buying a replacement lamp from Woolworths was still easy, knowing it would work when you get home. The lights were cheap, easy to select and fit, but they were not very energy efficient.

The source of confusion:  A fundamental problem is that historically most people have used ‘Watts’ as a measure of how bright a light is. Whereas, ‘Watts’ is actually a measure of power consumption. If you ask a photographer about light measurement, they will know that ‘Lumens’ and ‘Lux’ are the measurements of light intensity, not Watts.

So we need to understand the relationship between Power(Watts) and the light(Lux/Lumens), and the fact that modern energy saving lamps get more ‘Lumens per Watt’ than older inefficient lamps.  Additionally LED lamps also bring the colour temperature of light into greater relevance, e.g. ‘Warm or Cool white’.  Which means that multiple interrelated factors must be considered when selecting the best light for a particular application.

The following brief descriptions are intended to un-cloud some of the mysteries of modern lighting, specifically; Watts, Lumens, Lifespan, Colour temperature, Beam angles, Dimmers, Cap styles (bayonet, screw, GU10, MR16, G9, G4 etc).  If you are unfamiliar with any of these terms, a quick read may assist with future lamp selection.

Watts:  this is the amount of power a lamp will consume whilst producing light. Watts is Voltage x Current. Bigger watts means more current, more current means more heat. With lighting more heat = WASTE. we need light not heat from a lamp.  Because of the heat generated by higher wattage lamps many lamp holders and lamp fittings will have a maximum watt rating quoted.  This may be due to limitation of the fitting itself, the shade around the fitting or if it is an enclosed or an open lamp. (enclosed lamps can trap the heat generated by the light so may be lower max wattage rated fittings.)

Lumens and Lux:  these relate to the amount of light generated, (how bright a lamp is), but we all know that light gets duller the further away from the source so distance as well as brightness needs to be taken into account. Historically we used measurements called foot candles, (basically the brightness of one candle measured one foot away.)  Lux is the metric measurement of lumens per square meter.  1 Lux = 1 lumen per square meter.  {one Lux is approx. 0.093 foot candles}

Fuel efficiency

We are probably all familiar with the concept of fuel economy in our cars wanting more Miles-Per-Gallon. The equivalent fuel economy with lighting is wanting more Lumens-Per-Watt. Modern lamps should be achieving a minimum 40 lumens-per-watt to be considered energy efficient.

Most people have not been used to knowing how many lumens they are getting for their watts, because the manufactures used to put the watts in big writing on the front, but the lumens was in small writing hidden on the back or side of the box! A couple of examples to illustrate this point:B&Q CANDEL1B&Q CANDEL 2

A B&Q box of four 40watt candle shaped lamps. On the front the power consumption rating, (fuel in), is clearly written as 40W.

But the lumens, (light out), is in tiny writing last but one line from the bottom on the rear of the box.  360 Lumens!

360/40 = 9 lumens per watt.

{you may need to zoom your screen to read the small lumens text!}

WILKO CANDEL1A similar Wilkinson box of four 60 watt candle bulbs.   WILKO CANDEL 2

Again the 60watt is nice an big on the front. But this time we have to look onto the side panel to find the light output.

In this case it gives output as 600 Lumens.  (bigger writing than the B&Q example but still small!).          600/60 = 10 lumens per watt.

Not all manufactures actually write the lumens anywhere on their boxes. But as a very rough rule-of-thumb, with most traditional GLS filament lamps you get about 10 lumens per watt. So 60 watt lamp is approx 600 lumens, 100 watt lamp is approx 1000 lumens.


DIALL BOX 1Things are getting better. A more modern B&Q own brand box of 42watt halogen lamps. 625 lumens is clear on the front, and the 42 watt is smaller.

625/42 = 14.9 lumens per watt. Still not a true energy efficient lamp but a bit better.  (They claim this is equivalent to a 54w lamp, )

And a Wilkinson compact fluorescent lamp, 715 lumens nice and big, 12 watts much smaller, 715/12 = 59.6 lumens per watt. A more reasonable energy efficient lamp. (This is claimed to be equivalent to a 58watt lamp.)


Colour and Duration

Expected lifespan:  As well a having good Lumen-Per-Watt energy efficiency we also need good lifespan duration, especially when the lamps are more expensive!  The lifespan is normally written as thousands of hours, (e.g. 1000h or 1000hours  again small text).  Traditional lamps had 1000 or 2000 hours, or in some cases ‘long life’ lamps quoted 3000hours.  Energy efficient lamps typically start at 10,000 hours up to 50,000hours.  {As a point of reference remember 365days x 24hours = 8760hours per year. }  So a light that has an expected lifespan of 20,000 hours could theoretically be left switched on continually for 833+days or 2 years and 103days. But if you allow for an assumed number of hours per day use, say 5 hours, Then the lamp could last for 4000 days or just under 11years.

The lifespan figures are calculated by manufactures using a sample of lights in a test lab. So if the bulk of the lamps fail after 2000hours they will be defined as 2000hour lamps, even though a certain percentage will fail much quicker and some will last much longer. In the real world other factors can reduce the average lifespan of lamps;  Bumps and knocks during transportation. Electrical surges and fluctuations of supply. Extreme heat or cold. Enclosed light fittings (reduced air flow around lamp). Switching events, some lights deteriorate faster if repeated on/off switching occurs.


Colour temperatue, Kelvin: Anyone who enjoys shopping for new clothes will be well aware that artificial light can make colours look different to outdoor daylight. Not all lamps create light of the same colour temperature. Traditional filament lamps are a more warm glow light. Some modern energy saving lamps can have a blue, cooler light. Light temperature is measured in Kelvin, (k). This is another number that is written onto new lamps.

The B&Q Diall lamps illustrated earlier have a number and a small colour chart on the back showing the Kelvin rating as 2800k. Some manufactures just describe their lamps as Warm or Cool light.


A lot of internet suppliers have technical information pages giving greater detail about lighting, for example has a chart about colour temperature. It can be found at  it can be seen that 2700k or lower are generally warm looking lamps,   3000k or higher are colder looking lamps.

Cool lamps can be effective in bathrooms, showers, hallways, study/office, garage/workshop where a bright crisp light does not look out of place. Whereas lounges, bedrooms, dining rooms need warmer lighting to feel more relaxing and cosy. Kitchens can be a bit of a both warm or cool area, nice to be cosy and warm feeling but can be good to have clear crisp light over work areas while preparing food.

The colour temperature is a very important factor when selecting LED lighting, as these tend to be far more clinical and cold looking light compared to filament lamps, halogen or even some compact fluorescent lights.


Spotlamps and Beam Angles


A traditional lamp hanging from a ceiling pendant rose emits light in a 360º pattern, with only the base fixing of the lamp and any lamp shade restricting the light.  Whereas recessed ceiling spot lamps, typically GU10 or MR16 fittings have a flat face, where light is emitted in a beam pattern similar to what you get with a hand torch.

So in addition to the lumens and colour temperature the angle of the beam needs to be considered for the application of where the lamp is to be used. A narrower beam will give greater variation of Lux levels on surfaces inside and outside of the beams footprint. LED5W PIC1LED6W PIC1LED4.5 PIC1These types of lamps will normally have the beam angle written somewhere on the packaging. Some can be as narrow as 35º others as wide as 120º, which can make a very big difference to the lighting effect in a room. Other factors influencing the choice of beam angle include; the amount of lamps fitted in a room, if they are for highlighting a feature or just general room lighting.

On three example GU10 LED lamp boxes illustrated below we can see;  a 4.5watt 380 lumen with 120º beam angle,  a 5watt 480 lumen with 90º beam angle and a 6watt 580 lumen with a 60º beam angle.  (If we calculate the lumens per watt they range from 84 to 96, so relative to the traditional filament lamp you get 8 or 9 times more light for the same power consumed.)

                                      LED4.5 PIC2          LED5W PIC2          LED6W PIC2

Dimming and Dimmers

We are all familiar with using dimmer switches to help set the mood and atmosphere of a room, but may not be aware that not all dimmer switches can work all types of light, and not all lights are dimmable.

Max and Min power rating: A dimmer switch has a maximum load in watts that it can control. Too much load, (too many lamps), will either reduce the life of the dimmer or burn it out completely. The types of dimmers that have been common use in domestic properties for many years have a typical max rating of up to 250watts or 400watts. So if you tried to run six 60watt lamps, (360watts), with a 250watt dimmer switch it is not going to last very long.

So theoretically with our newer low energy lamps things should be easier. Unfortunately dimmers also need a minimum load to work correctly,  a traditional dimmer may have been rated as 60watts minimum load. Below this load and lamps could start flickering or not working at all, so a single 40watt lamp is also no good for a 60w->250w dimmer.

If you have changed four 50watt spot lamps for new 4.5watt LED’s, what used to be a 200watt load, (ideal for a 60w->250w dimmer), is now an 18watt load. Too small as 18watt is below the 60watt minimum. On top of all that, the electronics of how the dimmer actually controls the lamp needs to be different for LED’s  AND not all energy saving lamps are dimmable anyway!

So if you are looking to replace old lamps with low energy lamps in rooms controlled by a dimmer switch you will probably need to get a new switch compatible with the type of lamps you are fitting, assuming you have actually purchased dimmable lamps.  On the three LED boxes above it can be seen that the 6watt (right hand side), is a non-dimmable lamp.     {modern LED compatible dimmers will typically work from 10watt minimum load.}

Cap style, (how the lamp connects to the light fitting)

Once you have got your head around all of the above bits, the only other thing is making sure that you have the correct style of lamp connector so it can physically fit into your light. Bayonet and screw fittings are the two traditional styles, with each having a large and small diameter version.


Bayonet:  May be described as; Bayonet, Bayonet cap, Small bayonet, Miniature bayonet, B, BC, B22, BC22, SBC, B15,  BC15 etc..

These lamp holders are quite robust and reliable, but the two small sprung pins which have to makeBC LEDB15 HALO BCcontact with the lamps electrical terminals can deteriorate over time due heat generated by the lamp. They can become stiff so they do not spring up to connect with the lamp, so a new lamp fails to work in that holder. The plastic around the shroud that holds the lampshade in place can become very brittle over time.


Screw thread:  May be described as Screw, Screw cap, Edison Screw, Miniature Screw, Small Edison Screw, S, ES, E27, SES, E14 etc..

These lamp holders are again reasonable fail safe, but they do have two smallES metalES R63 SPOTESHALOSESMINILED terminals, which should make sprung contact with the bottom pin and side thread of the lamp for it to work. When the lamp holder is getting old or the lamps have been left turned on for long periods heat and wear and tear can cause the metal terminals to fail to make contact with the lamp. Occasionally the thread can stick between the lamp and holder, causing the complete lamp holder to rotate when trying to unscrew and old lamp, or cause the glass body of the lamp to separate from the screw base. Great care should be taken removing old lamps that are stuck in the holder, as twisting the lamp holder can cause the wire connections to break off or short out or touch the inside of metal light fittings.


MR16:  Small push fit lamp holders for low voltage lamps with two small pins.

These lights are not 230v, they are only 12v and somewhere in the fitting or the supply wiring there will be a transformerMR16 LAMPHOLDER3 or LED driver to provide the correct voltage.  If the wiring from transformer to the light is too long, or there are too many lamps off one transformerMR16LEDMR16HALOMR16 then the lamps will not operate at their full brightness due to reduced voltage.

They have a small push-fit connection which is undoubtedly a week point in the whole design. Over time the quality of the contact between the lamp and the connector deteriorates to the point of failure. Probably 90%+ of faults relating to spot lamps not working are due to the failure of these connectors. (Or if not connectors its the transformer).

There are certain ‘zones’ within bathrooms where 230v lighting is not permitted, so these 12v are one possible solution, but as a general rule if looking for an equivalent physical size lamp a 230v GU10 option is a more reliable solution.


GU10 & GZ10:  230v connection for 50mm spot lamps.

Two very similar type lamp connections, GZ10 has a squarer edge and the GU10 has a small  bevel angle. GU10 LAMPHOLDER2The difference is to ensure the wrong type of lamps cannot be installed in the wrong light fitting.  GZ10 lamps are classed as cool beam because they radiate more heat through the back of the lamp  and thusGU10 theGU10HALO GU10LEDfitting has to be capable of dissipating this heat. GU10 lamps are more like a reflector spot lamp and throw light and heat forwards. Their fittings are not designed for the sort of heat that a GZ10 would radiate backwards.

GU10 lamps can fit in a GZ10 holder but not vice-versa. As a general rule the GU10 lamp is the more common and is the one you are most likely to find when purchasing these spot lamps. Most LED’s of this size have the GU10 base. The fitting has  two small keyhole shaped slots, the lamp is inserted then turned to lock it in place. They are a very robust and reliable lamp connector.


G9:  A 230v connection small push fit lamp.

These G9 LEDG9 HALOGENlamps are bullet or capsule type bulbs that simply push into the lamp holder.  They are generally a reliable type of connection.

Often used in external spot lamps or decorative internal lighting,  especially the type with numerous spindly arms or branches coming out from a base.


G4: A 12v connection small push fit lamp.

These lights are 12v , not 230v, so they will also have an associated transformer either built into the light fitting or in the supply wiring. They are a bullet capsule type of lamp with two small pins for the electrical connection.  Often used in decorative lighting in G4 HALOGEN CAPG4 LEDwardrobes, glass display cabinets, kitchen units, bathroom mirrors and cabinets etc.

As with the MR16 push connector these can be prone to failure due to poor electrical contact. Can be a useful solution in bathrooms where 230v lights may not be permitted inside zones, but more often than not they can be more hassle with lamp failure problems. Over time due to heat, the connections lose their tension to maintain a good grip on the lamp, so if you hold or wiggle the lamp it may come on, but once you let go it goes off. At this stage it is generally cheaper to replace than try to repair.


R7S:    Linear stick shape lamps.

The traditional halogen garden flood light lamp. Two common lamp lengths being 114mm and 80mm. They are a relatively wasteful method of lighting, they consume a lot of power, generate a lot of heat and don’t produce many lumens per watt. R7S 114MM [ Two sample ratings are;  400watt,  9000lumen (22.5 lumen per watt),   80mm  120watt, 2300lumen  (19.1 lumen per watt) ]

These lamps are often sold cheap and fitted as DIY tasks, in inappropriate locations, causing light pollution over adjoining properties.  The lamps are cheap but expensive to run, a 400watt lamp will consume one unit of electricity every 2.5 hours, a 120watt lamp will consume one unit every 8.3 hours. Whereas a 30watt LED floodlight will run for over 33hours, or a 20watt LED over 50 hours per unit.  For several years there have been far better solutions than using these type of lights and I would not recommend them being chosen as a new light.

It must be noted that some modern LED light fittings have built in LED’s that are not replaceable.


To try and do a comprehensive list of all combinations and permutations of lamp types, shapes and fittings would be impossible, but most good online lamp suppliers have a lamp shape or type search when shopping.

Lyco have a couple of useful pages listing example shapes and fittings.  By cap style:    Or by shape:

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 Posted by at 7:31 pm