What are the different wattage sizes of light bulbs?
Wattage sizes describe how much electricity a bulb consumes, not how bright it is. Modern LED bulbs come in a wide range of low wattages, and older incandescent and halogen bulbs ran at higher wattages to produce similar light. To compare bulbs, check the lumen rating instead.
The first thing to understand is that a watt is a measure of power consumption. In the days of traditional bulbs, people used wattage as a rough stand-in for brightness, because all incandescent bulbs worked the same way. That habit stuck, but it no longer makes sense. Energy Saving Trust points out that watts measure power consumption rather than brightness, and because energy saving bulbs use fewer watts, it is better to look at lumens when shopping.
[1]Why the old wattage ladder is misleading
Two bulbs drawing the same number of watts can produce very different amounts of light depending on their technology. LEDs are described by Energy Saving Trust as by far the most energy efficient bulb type, matching halogen lights while using around 80% less electricity to do so. Incandescent bulbs were particularly wasteful, converting only about 5% of the electricity they used into visible light, which is why the UK Government banned their sale in 2021. So a low-wattage LED can easily out-brighten a higher-wattage older bulb.
[1]One honest limitation: wattage-to-lumen conversion figures vary by source and bulb generation, so no specific numbers are reproduced here. What Energy Saving Trust does publish is a table comparing the wattage of traditional bulbs with approximate equivalent lumen values for LEDs and CFLs, and that kind of table is the reliable way to translate an old wattage habit into a modern lumen choice. Check the current version of that guidance rather than relying on remembered figures.
[1]A worked example
Imagine a hallway with an old spotlight fitting. You remove a halogen bulb and want an LED replacement. The wrong approach is to hunt for an LED with the same wattage, because the LED will use far less power for the same output. The right approach is to note the lumens on the old bulb's packaging or fitting instructions, find an LED with a matching lumen rating, confirm the cap type fits, and check dimmer compatibility if the hallway light is on a dimmer control. Energy Saving Trust notes that not all LED bulbs work with a dimmer, and an incompatible dimmer may need upgrading to an LED-compatible model, ideally confirmed with an electrician.
[1]A checklist before you buy
- Match the fitting and bulb cap to your light fixture.
- Check the lumen rating for brightness rather than the wattage.
- Check the colour temperature, with warm white around 2700 to 3000K suiting most general household lighting.
- Confirm dimmer compatibility if the bulb will be used with a dimmer control.
- Confirm outdoor suitability if the bulb is going outside.
Colour rendering can matter too. Some bulbs distort how other colours look compared with natural light, and the colour rendering index, or CRI, shows how accurately a bulb illuminates colours. A higher CRI means more accurate colour appearance under the light.
[1]One more point on running costs: lighting makes up only a small part of the average UK household energy bill, so while switching to LEDs is one of the easier and less expensive upgrades, it is not a dramatic saving on its own. Smart bulbs offer extra control and colour options but, as Energy Saving Trust notes, they are not likely to save energy.
[1]When you have settled on a lumen target, cap type and colour temperature, compare the exact same bulb model across retailers before buying, checking the final delivered price and pack contents with each seller.
[2]Sources and dates
- https://energysavingtrust.org.uk/advice/lighting/ · accessed 2026-10-18
- https://lightingcompared.com/ · accessed 2026-10-18