5: Assessing lighting savings

Overview - More detail

Having calculated the existing lighting density, it is relatively simple to assess the potential for efficiency improvements. Here are the steps.

Step 1.

Identify the correct illuminance. Often the easiest lighting energy savings arise from reducing excessive lighting. The current Australian Standard mandates 320 lux for general offices, 240 lux for storage and 80 lux for toilets and corridors.

Step 2.

Calculate your lighting power density, as described in the previous section.

Step 3.

Calculate the target power density level. This is the lighting density that could be achieved using triphosphor tubes in normal luminaires. This is 11 W/m2 for open-plan offices and 18 W/m2 for small offices at 320 lux. For different lux levels, adjust in direct proportion to the change in light level. Some correction factors may need to be applied to these figures to allow for lower efficiency diffusers, dark dŽcor or improved ballasts.

Step 4.

The energy savings potential is the difference between the actual and target lighting power densities, multiplied by the expected hour of use.

Step 5.

It is worthwhile to check current lighting levels against accepted design standards. If current light levels are satisfactory, then the estimated savings can probably be achieved. If not, additional work may be required to bring the levels up to standard and this may affect savings.

Assessing lighting savings

Identifying and realising the potential for improving lighting efficiency requires a number of actions:

An enthusiastic energy manager with some technical knowledge can often carry out the first two items. This section provides step-by-step instructions for these actions. Professional help may be required to implement the retrofit.

Step 1. Identify the required illuminance

This first step may seem self-evident, but many a lighting installation has been designed with expensive and sophisticated lighting efficiency equipment such as electronic ballasts, mirror reflectors, daylight control and so on but is designed to deliver twice the illuminance required by the Australian standards. Even with all this sophistication the lighting system will use more energy than a standard design that provides the correct lighting level. Consequently selecting the correct light level can be your biggest energy saver. Select light levels from the illuminance table.

Space Maintenance illuminance
General office tasks and reference material of average detail.
Entrance foyers and reception areas.
320 lux Measured on a working plane 750 mm above floor level
Filing and storage rooms, walkways in open plan areas 240 lux Measured on a working plane 750 mm above floor level
Passageways and toilet blocks 80 lux Measured at floor level
Some maintenance illuminances from AS 1680.2.2 1994

These illuminances must be maintained up to the point where maintenance of the system is required. This means that initial light levels will usually be higher than the levels shown here.

Step 2. Calculate your lighting power density

The next step is to calculate the actual power density in your office as described in the previous section.

Step 3. Calculate your target lighting power density

To calculate energy savings you need to have some estimate of the ideal power density. The table below allows you to assess the power density required to light office spaces using normal luminaires with triphosphor lamps that are cleaned regularly. Note that these power densities are based on a retrofit scenario and are not as good as can be achieved with a state of the art new design. By using the correction factors for different luminaires and tubes, the effectiveness of existing lighting and proposed replacements can be assessed in most cases.

320 lux 240 lux 80 lux
Small office 18 W/m2 14 W/m2 5 W/m2
Open plan 11 W/m2 8 W/m2 3 W/m2

Select the column corresponding to the illuminance you require. Then select a room size corresponding to the room you are interested in. (This doesn't have to be too exact. If looking at a whole building you can take an average depending on the proportions of the different room sizes.) Then read the power density required from the body of the table.

The table assumes "normal" conditions. To apply to some wider conditions, use the following correction factors.

Situation Correction Factor
Silver plastic grid diffuser 1.5
Dark walls and floor 1.15
Opal diffuser 1.10
Low loss ballast 0.90
Electronic ballast 0.80

Step 4. Calculate energy savings

Calculate the potential savings as follows. Use the following variables:

H = Hours per year. This is about 3000 for an average office.

Pexist = Existing lighting system power density in W/m2

Ptable = Lighting power density from the table in W/m2

C = Correction factor

A = Floor Area, m2

Esaved = H x A x (Pexist - C x Ptable )/1000 , in kWh

If you are interested in the effect of using different tubes, electronic ballasts etc then re-calculate the new power consumption using the appropriate correction factor.

Calculation of Energy Savings

Step 5. Check existing lighting levels

As well as calculating power densities it is desirable to ascertain current light levels. Actual light levels will vary by up to a factor of three, so it's important to take representative measurements.

The best we can do is to measure the light level in six to ten critical spots (usually on workstations) and see if the average falls above the maintained illuminance required. If the answer is yes and the lighting is reasonably old and dirty then the system is probably capable of delivering the maintenance illuminance required, efficiency not withstanding. For new systems, the light levels need to be some 20% higher than those in the tables, to allow for accumulated dirt and degradation. If light levels appear satisfactory then the savings calculated using the power density table can probably be realised.

If light levels appear to be unsatisfactory, then either the luminaires are extremely dirty or deteriorated or the system is under-designed and improving the light output will detract from the energy savings available.