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Low-energy bulbs: specification and performance

Ranges of low-energy (or 'energy-saving') bulbs are changing all the time, making it difficult to decide what type will work best in a particular setting. Naturally, if you're spending £5 or even £10 on a bulb, you want to know the answers to questions such as how bright will it shine, will light it up straightaway, and how long will it last? The B&Q website has a Buyer's Guide to Light Bulbs, which you may find useful.

Label from package for a typical LED 'energy saver' bulb
lightbulb label

Regulations introduced in 2010 mean that manufacturers now give much more information on the packaging about a bulb's performance. But just what do all the numbers mean? It will take time for such information to become widespread, so if in doubt, try asking the retailer!


The different types of fitting are each given a code; failing that get an assistant to open the box so you can be sure!SES screw fitting

  • BC or B22: bayonet cap, standard size
  • SBC or B15: small bayonet cap
  • ES or E27: Edison screw fitting, standard size
  • SES or E14: small Edison screw
  • GU10: turn and lock two-pin fitting (e.g. for spot lamps)

Adaptors are available to enable, for example, a GU10 bulb to be used in a conventional bayonet fitting.

GU10 fitting

Small Edison screw fitting

Light output (brightness)

This is measured in lumens (symbol: lm). For example, a traditional 60 watt incandescent bulb produces about 600-700 lumens of light. Roughly the same amount of light should be produced by an 11 watt CFL bulb, or by a 40 watt halogen bulb.


Close-up of GU10 fitting for 240V LED bulb

Warm-up time (or run-up time)

This is the time the bulb takes to reach 60% of its peak light output. The categories are:

  • fast: less than 30 seconds
  • standard: 30-60 seconds
  • slow: 60-120 seconds

Do not confuse the 'warm-up' time with the 'starting time', which is the brief time taken for the lamp to switch on initially, usually little more than 1 second. Hence a bulb can be described as starting 'instantly' but still be 'slow' to warm up. LEDs generally start up more or less instantaneously.

halogen bulbA standard 40W halogen bulb is equivalent to a traditional 60W bulb

Colour temperature and colour rendering index

This is measured in kelvins (symbol: K). For sources such as a CFL bulb, the correlated colour temperature (CCT) enables comparison of the quality of its light with that from traditional incandescent bulbs. The latter have colour temperatures usually in the range 2700-3300K, and so low-energy bulbs with values at or below this range are perceived as having a warm or soft tone, whereas those above this range will look whiter or 'cooler'. CFL bulbs span a wide temperature range; some are designed to mimic the tone of a traditional incandescent bulb. Halogen bulbs tend to have a 'crisper' whiter light. LED lamps have colour temperatures on a par with old-style bulbs.

The colour rendering index (CRI), often symbolized by Ra, is a measure of how faithfully the light source reproduces the colours of objects compared to daylight. Values lie on a scale from 1 to 100, with those approaching 100 indicating the highest degrees of faithfulness.


This is given in hours, with 1000 hours generally assumed to represent 1 year's use (at roughly 3 hours per day). The lifetime of halogen bulbs is typically 2000 hours, whereas CFLs can range from 6000 to 15,000 hours or more. LED lamps have the longest life expectancy, with some designs rated at 25,000 hours.

Energy labelling

Household bulbs are labelled according to their energy efficiency. The classification system grades bulbs from A to G, with class A having the highest efficiency.

Lumens, watts and energy efficiency

Light is measured in lumens, and power in watts. The lighting efficiency of a bulb is thus expressed in units of ‘lumens per watt’ (LPW). Traditional incandescent bulbs give 12-20 LPW, with bigger bulbs producing more LPW. Energy-efficient bulbs have much higher LPW ratings, between 50 and 70 LPW – which is why an energy-efficient bulb rated at a lower wattage produces the same amount of light as a higher rated incandescent bulb. For example, a 25W CFL that produces 70 lumens per watt will emit 25 x 70 = 1750 lumens, about the same as a 100W incandescent bulb. In other words, in this example an energy efficient bulb uses one quarter of the power to give the same effect!


The “Energy Saving Recommended” logo from the Energy Saving Trust means a bulb has met strict criteria and been independently tested. As well as energy consumption, the criteria include:

  • How long the bulbs last – this is usually marked on the box, e.g. 10 years lifetime based on 10,000 hours of use. Even for good quality bulbs, expect some dimming over the lifetime of the bulb.
  • Quality of light and warm up time – the best quality energy saving bulbs now use electronic ballast (the bulky bit at the bottom) rather than magnetic ballast. This makes them smaller and lighter, eliminates the problem of flickering and humming and reduces the time to reach full brightness, as well as producing a better quality of light.