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How we test energy saving light bulbs

By Matthew Knight

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How we test energy saving light bulbs

No one tests light bulbs as thoroughly as Which?. Read on to find out about our extensive light bulb testing.


Our Best Buys light bulbs are efficient, durable and won't degrade over time, and the colour and type of light they produce closely matches what is stated on the packaging. 

Don't Buy bulbs on the other hand, have disappointed in our tough tests by being inefficient and producing a poor or inconsistent quality of light that doesn't match up to the claims on the packaging. We also check that each bulb passes electrical safety tests, and any that fail are automatically labelled a Don't Buy.  

We test three different types of LED light bulbs: standard LED bulbs (B22/E27 classic globe fitting), filament LED bulbs and LED spotlights (GU10 fitting). We put each one through tough testing under controlled lab conditions so we can recommend the light bulbs that truly shine. 

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We go to great lengths to design thorough and rigorous tests to answer all of the most important questions about light bulbs, including:

  • How long do the bulbs last?
  • How efficient are they?
  • Are they safe?
  • Are they as bright as they claim?
  • Does it produce the right type of light?
  • Is the light consistent?
  • Should I buy it?

We test all of these things and many more. See below for a more detailed explanation of each test and our additional tests for spotlights. 

How durable are the light bulbs?

In 2016, we changed our approach to testing durability. We've spent the last three years continuously testing large batches of LED bulbs to simulate their claimed lifetimes of more than 15,000 hours. We found that they very rarely failed before this time, and that LED bulbs are generally reliable and live up to their lifetime claims.

However, we do still test durability for newer types of LED bulb, where the technology is significantly different, to ensure that you won't get stung by bulbs failing long before they claim to. 

Filament LED durability 

As filament LEDs are still relatively new, and the way the LEDs are arranged is different to standard LED bulbs, we test all filament LEDs for durability. As part of our durability testing we:

  • Leave each bulb switched on for 2,000 hours (equivalent to two years of use) to see whether the light bulb will run the course in your home. We've found that if a bulb is going to fail, it usually fails within this time frame.
  • Measure the brightness of the bulb after it has been switched on for 2,000 hours, to see whether the light bulb gets dimmer over time.
  • Switch three samples of each light bulb on and off 30,000 times to test whether it will cope with heavy use in your home.

Which bulbs are energy-saving?

The more efficient the bulb, the lower your energy bills. So to save money on your electricity bill, we measure how many lumens each bulb emits and then calculate how many lumens each bulb produces per watt. The least efficient will only produce 13 lumens per watt; the most efficient can produce up to 102 lumens per watt. 

Are they as bright as they claim?

A common complaint about energy-saving bulbs is that many grow dimmer over their lifetime. To check whether a light bulb will fade, we measure how bright each light bulb is after its been switched on for 100 hours and then again after the bulb has been on for 2,000 hours. 

We then compare this level of brightness against the manufacturers' claims on the packaging. We have found bulbs that are more than 20% dimmer than advertised.

We also test how much energy the light bulbs use, showing us how efficient they are and how much they will cost you to power each year.

For the newer type of LED bulbs, filaments and super bright 100W equivalents, we also make sure that the bulbs are still as bright as they claim after our durability testing. 

Are the bulbs safe?

We also check that the light bulbs will protect you from electrical shock and have adequate electrical insulation. We've found some bulbs that failed these tests in recent years. When we do identify an unsafe bulb, we follow this up with the manufacturer and trading standards to ensure they don't pose a danger to the public. 

Do the bulbs give off good quality,warm white light?

When it comes to the colour of light, many of us are used to the warm, white glow of a traditional incandescent light bulb. Our graphic, below, gives you a better idea of how the colour temperature of light is represented on the Kelvin scale. An incandescent bulb is around about 2,700K, known as 'warm white'. 


We measure the colour of the light from each of the bulbs that we test and see how it compares with what's specified on the bulbs' packaging. 

We also test each bulb's Colour Rendering Index (CRI) value. The CRI is a rating that describes a bulb's ability to reproduce the true colour of an object, for example, how red it makes tomatoes look. This is factored in to the overall test score. 

We combine these scores to give an overall rating for the light quality, so you know your bulb with give off a consistent glow.

Is the light consistent?

A common issue with LEDs is that some are an inconsistent colour temperature, so even if you buy the same bulb, you may end up with subtly different hues in your home. We test up to five examples of the same bulb to make sure that the brightness and light quality is consistent for each model, so that you won't take your bulbs home and find that one bulb is obviously brighter or a different colour to another in the same fitting.

Does the light degrade over time?

We test the colour and and the colour rendering of the light when the bulb is brand new and then again after we have simulated two years of use. The best bulbs will maintain their colour and colour rendering over time, whereas the worst will degrade in quality.


Many people choose spotlights based on the specified beam angle on the box. We test each spotlight against these claims and make sure that the right amount of light falls within the specified cone angle.

The brightness test for spotlights also takes account of how much light falls within a useful 90-degree cone and whether any light spills out beyond this. Halogen bulbs tend to give a more diffuse light than LEDs, which can achieve a very sharp, narrow beam angle. Visit our guide to buying spotlights for more information on what to look out for.

Should I buy it?

All of the above tests and more are analysed and feed in to create an overall test score and recommendation. Only the very best light bulbs get a Which? recommendation and are classified as .

For standard LED light bulbs and filament LEDs the total test score ignores price and is based on:

  • 45% efficiency
  • 30%  light quality
  • 20% light output
  • 5% power factor

The power factor indicates how closely a bulb meets its power usage claims.

For spotlights the breakdown is:

  • 40% efficiency
  • 25% light quality
  • 15% light output
  • 15% beam angle
  • 5% power factor

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