How many downlighters should I use?

If you currently have standard pendant lighting or wall lights and are converting to downlighters then it is important to get the right amount of spot lights so that the space has the correct light level. It is especially critical with the recessed spot lights for instance halogen MR16 bulbs as these involve cutting round holes in your plasterboard ceiling and this means that relocating them would be a major job. So make some calculations and get it right first time! The other consideration is that if you are starting with say 50W halogen downlighters and planning to migrate to LED Bulbs for energy efficiency reasons, then the LED bulbs are generally not as bright so you will invariably need to use more.

Downlighter Spacing Calculations

If a lighting architect was  to do a true theoretical calculation then he would start with the LUX level required for the room and from this work out the lumen’s value needed from the luminaires. This is quite complex because you need to take into account a whole range of factors like wall colours and  the directional aspects of the lighting. Then after deducing the lumen’s needed  the specific light bulbs can be selected. This may sound great but the issue is that most of the variability comes with deducing the lighting level variation caused by factors such as the wall, work surface and flooring colours which means that as a result the calculation is only an indication. Accordingly  I would propose that you use some rule of thumb indicators to work out how many downlighter are needed.

The first rule of thumb is that for a normal living area you need approximately 1 watt of light per square foot and for a work area where clarity of illumination is important,  for instance a kitchen, them it is best to have between 1.5 and 2 watts per square foot. Now this wattage is assuming that you use the normal halogen downlighters and not low energy halogens or CFL or indeed LED bulbs. Clearly if you had 1 watt per square metre of LED light then the room would be dazzlingly bright!

Calculation of the number of lights needed  for a Living Area

If we take a large kitchen, breakfast room area of size 5 by 7 metres then at 1 watt per square foot this would mean that you need 377 watts of lighting. This could be accommodated by using different configurations.

1. Use 8 x 50watt dowlighter in 2 rows of 4 giving 400 watts of lighting.

2. Use 12 x  35 watt downlighter in 3 rows of 4 giving 420 watts of lighting.

3. Use 15 x 25 watt downlighters in 3 rows of 5 to give 375 watts of lighting.

This gives you a starting point but then you need to take into account other factors to tune the number.

If you have a higher than average ceiling height then it is best to increase the number slightly as downlighters illuminate best at a height of about 1.5-2 meters. The other important consideration is what other lighting you have in the space. For example if you have under cupboard lighting and a cooker hood light then you can reduce the number slightly. Also as discussed earlier then the wall, floor and work surface colour will make a significant difference so if you have darker mat  colours in your design scheme then you will need to increase the illumination level.

The  other rule of thumb cross check is that the saving will typically be  between 1.4 and 1.6 meters for the 50watt equivalent downlighters which should enable a cross check to be performed before those holes are made in your plasterboard. So that is it,  but as a closing remark with my eco hat on then you need to convert the halogen wattage levels to LED or CFL equivalents. So a 25watt halogen is broadly equivalent to a 3 watt LED and a 7watt CFL MR16 is broadly equivalent to a 35watt halogen.

 

Calculation of the number of lights needed  for a Kitchen Area

So if we take the kitchen areas then it would be appropriate to use the equivalent of 15 x 50w halogen downlighters. This means that for the size of 5 x 7 m then 3 rows of 5 would give a high level of illumination suitable for a work area.

This equates to a closer spacing of between 1.2 and 1.25m.

Now is you are using a good quality high output 5w COB GU10 then then these can generally replace a 50w halogen on a 1:1 basis.

How many  Lumens Output per Square Metre are needed.

With the complication of LED technology always improving and the lumens output per watt going up with the new COB LEDs it is often more meaningful to use lumens as the base measure for light output. Using the calculation from before with our 5 x 7m room we get the following results:-

For a living area level illumination then approx 4000 lumens for 35 square metres which is 115 lumens/ metre.

For a work area level illumination which requires very bright light then this should be increased to 7500 lumens which equates to 215 lumens/metre.