Buying LED lights? What lumens, Kelvin, watts and CRI actually mean

You need a new light bulb. Simple enough, right? Then you look at the box and you see something like…

GU10. 4.7W. 345 lumens. 2700K. CRI 90. 36°. Dimmable.

Suddenly, buying a light bulb doesn’t feel quite so simple. Modern LED lighting is more energy efficient and gives us far more choice than older lighting technologies, but it also means there are more numbers and terms to understand. The good news is that most of them are actually quite straightforward. Here’s what to look for when you’re buying an LED bulb or light fitting – and what those numbers really mean.

Watts vs lumens: what’s the difference?

This is probably the most important change to understand when buying LED lighting. For years, we associated watts with brightness. Most people had a reasonable idea of what to expect from an old 40W, 60W or 100W bulb. But watts don’t actually measure brightness.

• Watts (W) tell you how much electrical power a light uses.
• Lumens (lm) tell you how much visible light it produces.

That’s an important distinction with LEDs because they can produce plenty of light while consuming relatively little electricity. As a rough comparison:

So, if you’re replacing an old bulb and want roughly the same brightness, compare the lumens rather than trying to match the wattage. And remember: more lumens isn’t automatically better. A bedside lamp doesn’t need the same output as a bright task light. The right brightness depends on where the light is going and what you’re using it for.

What does Kelvin mean on an LED bulb?

Once you’ve decided how bright you want your light to be, the next number to look at is Kelvin (K). Kelvin describes the colour temperature of white light. The slightly confusing part is that a lower number produces a warmer-looking light, while a higher number produces a cooler-looking light.

• 2700K – warm white
  A soft, warm-looking light similar to the traditional glow many people associate with older incandescent bulbs.
• 3000K – warm/neutral white
  A slightly cleaner, crisper white while still retaining some warmth.
• 4000K – neutral/cool white
  A cooler, brighter-looking white often associated with more practical or task-focused spaces.

So, which should you buy? There’s no single correct answer. Think about how you want the space to feel and what you’ll be doing there. A warm 2700K lamp might look fantastic beside a sofa in the evening but wouldn’t necessarily be your first choice above a workbench. Likewise, a crisp 4000K light that’s useful in a practical workspace may feel too cool for your bedroom.

What is CRI?

Another number you may find on better-quality LEDs is CRI, which stands for Colour Rendering Index. In simple terms, it relates to how faithfully colours appear under a particular light source. Why should you care? Think about food in a kitchen, clothes in a dressing area, artwork on a wall or the carefully chosen paint colour in your new living room. The quality of the light can affect how those colours appear. A higher CRI can be particularly worth considering where accurate and attractive colour appearance is important. It’s one of those specifications that people often overlook when buying lighting – but one that can make a noticeable difference to the finished result.

What does beam angle mean?

If you’re buying a GU10, spotlight or downlight, you may also see a figure expressed in degrees – for example, 36°. That’s the beam angle. It describes how widely the light spreads from the fitting. A narrower beam produces a more concentrated area of light, which can be useful for highlighting artwork, furniture or architectural features. A wider beam distributes the light over a broader area, making it more suitable for general illumination in many situations. This is why two downlights with similar lumen outputs can produce very different results. One might create a tight pool of light while another spreads that light across a much larger area.

GU10, B22 and E27 – what do they mean?

These aren’t descriptions of the light itself. They tell you about the lamp base or fitting. Some of the ones you’ll commonly encounter in UK homes include:

• GU10 – the twist-and-lock fitting commonly found in spotlights and downlights.
• B22 – the traditional bayonet cap found in many UK lamps and ceiling fittings.
• E27 – a larger screw fitting, sometimes called an Edison screw.
• E14 – a smaller screw fitting often found in decorative lamps and smaller light fittings.

You need the correct base for the fitting you’re putting the lamp into, so it’s always worth checking before ordering a box of new LEDs. Also bear in mind that many modern light fittings use integrated LEDs. In those fittings, there may not be a conventional replaceable bulb at all.

Can all LED bulbs be dimmed?

No – and this is an easy mistake to make. If you want to dim your lighting, look specifically for an LED that’s marked dimmable. But that’s only part of the story. The LED lamp or fitting and the dimmer control also need to be compatible. An unsuitable combination can result in flickering, buzzing, a poor dimming range or unstable operation. So if dimming is important to you – particularly if you’re installing several downlights or creating a new lighting scheme – it’s worth considering the controls at the same time as the lights themselves.

What about smart bulbs?

Smart bulbs add another layer of choice. Depending on the product and system, they can allow you to control lights from an app, change brightness, create schedules and, with some products, adjust the colour or colour temperature. They’re useful in the right situation, but don’t assume every light in your home needs to be smart. Before buying one, ask yourself what problem you’re trying to solve. If you want a lamp to switch on automatically while you’re away, a smart bulb might be useful. If you want several lighting circuits throughout a kitchen or extension to work together, a properly planned lighting-control system may be a better approach. Technology should make your lighting easier to use – not more complicated.

So, what should I look for on the box?

Next time you’re buying an LED light, you can decode the label in a few seconds. Take this example:

GU10 | 4.7W | 345lm | 2700K | CRI 90 | 36° | Dimmable

It tells you:

• GU10 – which type of fitting it uses
• 4.7W – how much electrical power it consumes
• 345lm – its light output
• 2700K – a warm-white colour temperature
• CRI 90 – information about its colour-rendering performance
• 36° – how widely it distributes its light
• Dimmable – it can be dimmed when used with suitable compatible controls

Once you understand those basics, choosing an LED becomes much less confusing.

The right bulb is only part of good lighting

You can buy an excellent LED and still end up disappointed if it’s in the wrong position, illuminating the wrong area or being asked to do a job it wasn’t intended for. That’s the difference between simply buying a light and planning good lighting. If you’re replacing one lamp, understanding the numbers on the box may be all you need. If you’re planning a new kitchen, extension, renovation, garden or complete lighting upgrade, it’s worth thinking about the whole scheme – including positioning, switching, controls and how the space will actually be used.

Choosing the right LED lighting? Let LAIRD Electrical bring your ideas to life.

Choosing the right LED lighting involves more than simply picking a bulb. Brightness, colour temperature, beam angle and dimming compatibility can all affect how a room looks, feels and functions. At LAIRD Electrical Contractors, we help homeowners across Leeds and Yorkshire create practical, energy-efficient lighting solutions tailored to their homes. Whether you’re upgrading existing fittings, installing new downlights or planning a complete lighting scheme, we’re here to help.

From modern kitchen lighting and feature lighting to outdoor illumination and smart lighting controls, we can help you achieve the right balance of comfort, functionality and efficiency. With competitive pricing, clear quotations and a 12-month workmanship guarantee, you can have confidence that your lighting installation is in professional hands.

Contact LAIRD Electrical Contractors today to discuss your LED lighting requirements, plan a lighting upgrade or request a competitive quotation.

Call 0113 4180 697 or tell us about your lighting or electrical project.