Lumens versus watts - Watt's the difference?

When you’re buying light bulbs nowadays, they’re almost certainly LEDs. They often measure their brightness in lumens as well as watts. They usually have an “equivalent wattage” measurement too, which refers to the typical light output of traditional tungsten bulbs.

It can all make buying bulbs a little confusing. LED bulbs obviously have a wattage, just as all electrical components do, but it’s much lower than the wattage of a tungsten bulb of the same brightness. We thought we’d clarify the differences and definitions here.

Wattage: a historical hangover

In the past, wattage was used as the only way to indicate the brightness of a bulb – the higher wattage, the brighter it was. This method became the standard way of describing bulb brightness for generations, as it was a simple way for consumers to understand and compare different tungsten bulbs.

However, watts are not a very accurate way of measuring light output. This is because the amount of light produced by a bulb is actually measured in lumens – the total amount of visible light emitted by a light source. Wattage is just the power consumption, and doesn’t directly predict the brightness. It’s a bit like measuring the fuel consumption of a car in engine loudness.

Remember that tungsten and halogen bulbs get very hot – and that energy has to come from somewhere. A lot of those watts were just heating up the bulb and not producing light. That is mainly why they were so inefficient. Also, if you think about it, a 40W bulb could be covered in a thick coloured tint so hardly any light would escape, but it would still be using 40W of energy. 

The advent of LED technology highlighted the limitations of using watts as a measure of brightness. LED bulbs use much less power to produce the same amount of light as a traditional incandescent bulb. For example, a 10 watt LED might produce the same amount of light as a 60 watt tungsten bulb.

Lumens to the rescue

Lumens (lm) gives a more accurate picture of a bulb’s brightness, as it’s literally measuring the visible light emitted. The amount of power used is irrelevant. Any light source, be it LED, tungsten, fluorescent or even a candle, has an output that can be measured in lumens. And a candle is using zero watts of electrical energy!

Brightness examples: lumens and watts comparison

To put it into context, here are some examples of equivalent lumens and watts for tungsten and LED bulbs. The tungsten bulbs are considered clear and non-tinted.

  • 25W tungsten bulb = 4W LED bulb = 220 lumens
  • 40W tungsten bulb = 6W LED bulb = 400 lumens
  • 60W tungsten bulb = 10W LED bulb = 700 lumens 
  • 100W tungsten bulb = 18W LED bulb = 1,300 lumens

Limitations of lumens as a metric

While lumens provides a more accurate measure of brightness than watts, it’s still not the end of the story as far as useful light is concerned. Lumens measures total light output without considering how effectively the light is directed or distributed. Light can be focused or widely spread out, so different types of bulb shape can have significantly different feels, even if they are the same in terms of lumens. For example:

  • Spotlights or GU10s: These bulbs typically focus their light into a beam or cone, and direct it towards a specific area, which is useful for accent lighting or task lighting. The concentrated beam means more of the luminous output is used effectively in a smaller space.
  • Traditional, golf ball or candle bulbs: These tend to spread their light more uniformly in all directions. That can result in less efficient usage, as much light is “wasted” by lighting the ceiling and walls, for example. That doesn’t necessarily mean the light is unwanted. Illuminating the walls and ceiling – or just the inside of a lampshade – has an ambient effect that is useful in itself. But if you’re planning a room with downlights rather than a pendant light, there’s a world of difference between 400 lm of focused light and 400 lm of light being sent in all directions.

Buy LEDs the smart way

Now you know the difference between lumens and watts for both incandescent and LEDs bulbs, and how the type affects the beam, you’re ready to buy your bulbs with confidence that you’re getting the right ones. Shop with ACAS for a wide choice of standard and dimmable bulbs in all the popular colour temperatures. We also have a great 30-day returns policy for extra peace of mind.