How do I know how many light fittings I need?

How do I know how many light fittings I need – Part 1) Downlights 

If you are remodeling a room and have the opportunity to replace your light fittings you might be wondering how many you need to put in. If you are looking to make the switch to LED how do you know what lamp you need to buy? How bright should your room be and how can that be translated into the lamps and fittings you are buying?

Back in the days of halogen it was pretty easy to work out. Usually you would put 50w halogens in a kitchen, 35w halogens or bedroom and 40w GLS lamps in your bedroom. The brighter the room needs to be, the higher the wattage you would use in that room.

LED has changed things, you can get better light out of a 3w LED than a 6w LED depending on the quality, beam angle and efficiency of the lamps and so we need a way to ensure that we have enough light output from our LED lamps.

 

Determining how much light you need in a room

The amount of light you need in a room depends on what you are doing. We measure the brightness of the light in a room with illuminance (lux) and different rooms have different recommendations. The average illuminance in the home is around 150lux.

The table below shows the rough lux levels for comfortable lighting in each area of the home:

Area of home Recommended Lux
Living Room 50-100 lux
Kitchen 300 lux
Bedroom 50-100 lux
Hall/Landing 150 lux
Study 300 lux

 

Worked example of choosing lamps for the bedroom.

Now we know how much light we need in the room we can determine how many lumens we are going to need. For this you will need to know how big your room is. I will take a bedroom of 4m x 3m. This gives us 12m2

The (rough) formula we are going to use for determining how many lumens we need is

Lumens = lux level x area of floor (in m2)

So for a bedroom that is on the darker side of comfortable I will be using a value of 50 lux.

So for lumens I need 50 x 12 = 600 lumens in total. That isn’t too bad at all, I can get 600 lumens out of 3 Crompton LGU103SMD 3W lamps (which have a wide beam angle – 90°, better for softer lighting) or some Aurora Enlite EN-GU1006 6w lamps (which have a smaller beam angle – 36°, more focused).

If you wanted to go traditional halogen you could always go for 2 or 3 35w halogen lamps but at 10 times the power consumption and a fraction of the lamps life it just isn’t worth it anymore!

 

Worked example of choosing lamps for the kitchen.

The kitchen needs to be brighter, there are few things as frustrating as trying to cook properly in a poorly lit room. Everything becomes much harder! The recommended 300 lux is over 6 times more than our bedroom example and you might thing that we are going to need a lot of lamps!

If we take a kitchen that is 4m x 2m (giving us 8m2) and we are aiming for 300 lux then our formula will read:

Lumens = 300 x 8 = 2400 lumens.

That seems quite a bit more but kitchens do need to be brighter and there will be other light in the room such as possibly having under cabinet or over hob lighting.

The Bell 05178 7w PRO LED dimmable lamps rate at 470lm each. We also have the Crompton LGU105COB 5w non dimmable lamps that rate at 450lm This would mean you would need around 5 LED lamps to get that kind of lumen output for your kitchen. As these are retrofit lamps you can fit them in any GU10 fitting such as the spotlights we have available from KSR.

Bell 05178 7w Pro LED
470 lumens (50w equivalent)

Crompton LGU105COB 5w LED lamp
450 lumens (50w Equivalent)

Aurora Enlite EN-GU1006 6W
240 lumens (35w equivalent)

Crompton LGU103SMD 3W
225-270 lumens (35w equivalent)

So why not just use the wattage?

Well hopefully you can see by the lamps above that the wattage isn’t always indicative of how much light you will get out of the lamps. The Aurora lamp (6w) is a higher wattage than the Crompton (5w) lamp but due to different LED technologies (standard vs COB) and beam angles (36° vs 45°) the lumen output is higher on the Crompton lamp.

The lumen output is important. Most lamps will have a lumen rating on them which you can use to determine if it is bright enough. If you are really lucky, the lamps will have LUX on them and there is no conversion needed!

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