Lighting Economist Dave Tilley turns his attention to a specific retrofit application this issue – warehouses. How do the alternatives to the ubiquitous HID low bays stack up?

How many commercial warehouses have been built over the years and illuminated with 400W HID low bays? I am not sure what the answer is, but at least 80 per cent of the warehouses I have visited over the past few months have this type of luminaire and lamp.

Before we look at the alternative luminaires and light sources it is worth considering the way a significant proportion of warehouses and distribution centres are constructed.

I am referring to the installation of skylights.

The key point is – can the daylight from the skylights be used to help cut energy consumption by introducing a ‘switching policy’?

Another point to consider – should consultants and installers advise clients to clean or upgrade their skylights? The lighting project could be lost if cleaning significantly boosts light levels.

I will leave you to form your own conclusions.

What are the alternatives to 400W HID low bays?

There are several candidate replacement sources for 400W HID low bay fittings:


The table below examines the return on investment of each of the options based on the estimated cost of materials and their life expectancy.

The calculations are based on an installation of 20 400W low bays with 20 per cent gear losses, operating for 4,000 hours a year. It is not possible to include installation costs in the analysis because there are too many variables, the most important one being the height of the installation.


So there is obviously some potential to reduce energy consumption by using alternative technologies. However, a simple mathematical calculation is not enough. In this case there are other important factors to be considered.

Three factors are crucial when reviewing warehouse lighting: lighting levels (in lux), lighting controls and zone definition.

Lux levels

Lux levels are directly linked to energy consumption. And the efficiency of the luminaires must also be considered – it will have a direct impact on the number of luminaires needed.

Luminaire manufacturers promote the benefits of a high light output ratio (LOR) on the basis that fewer luminaires with a high LOR (in per cent) will be needed in a given installation to achieve the same lighting levels – so installation costs will go down.

Makes sense, doesn’t it? On the face of it, a reduction in the number of luminaires and the corresponding reduction in energy consumption certainly appears to be a cost- effective proposition – but the installation cost may not be as low as manufacturers imply.

Point-for-point replacement of luminaires may be the only option when upgrading an installation, simply to avoid the cost of rewiring the entire warehouse. There may, of course, be a good financial argument for a rewire, but the potential scale of such an investment will usually negate this option.

Albeo Technology Constellation LED Low Bay

Albeo Technology Constellation LED Low Bay

An important factor to consider, as part of the installation, is the height at which the luminaires are suspended. Can the luminaires be suspended a metre lower? If so, the potential to reduce the wattage of the luminaires is increased.

T5 luminaires are flexible and can be configured with different numbers of lamps of different ratings. For example, four 80W lamps for a total of 320W, three 80W lamps for a total of 240W, six 49W lamps for a total of 294W or four 49W lamps for a total of 196W.

The range of configurations lets designers achieve a more balanced lighting installation.


Lighting controls will have a significant impact on energy consumption and – potentially – maintenance efficiency. However the introduction of lighting controls must be planned.

For example, if the warehouse has no natural light and is occupied continuously during operating hours, it is most important to ensure that the luminaires are turned off when the last person leaves. Whether this is handled by the lighting controls or with a ‘switching policy’ must be determined by discussion with the client.

In the vast majority of cases, natural light is present and warehouse space is not always occupied. Intelligent luminaires with occupancy detection, daylight detection and dimming functions will have a significant impact on energy consumption.

For example, the table below shows the effect of a combination of occupancy detection, natural light compensation and luminaire dimming, which results in a 50 per cent reduction in energy consumption.


The time to achieve ROI has fallen from 3.90 years to 1.90 years.


Zone definition is important because it will influence the lighting specification and design.

How many times have you seen a warehouse used not only for storage but for product assembly or similar activities? Where practicable the assembly area would be illuminated from a low level, close to the activity, while the storage space would be illuminated from a greater height.

Storage isles are often divided into frequent picking areas, towards the front, and infrequently picked items are at the rear. This kind of arrangement lends itself to individual and separate areas of lighting control.

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