Dimmers & Switchbanks
Dimmers & Switchbanks are both ways to control lighting levels, improving both utility and comfort.
Advantages of Lighting Level Control
Controllable lighting brightness permits
- Bright lighting when its needed for a task
- Relaxed lighting in the evening
- Improved safety by allowing low light levels to be used at night; particularly advantageous if small children are about.
- Give better comfort and better visibility.
- Alters energy usage
How it affects energy use and running cost depends on which method of control is used and the pattern of use.
Methods of Lighting Level Control
Switchbank
A Switchbank is a bank of 2 or more switches. If each switch controls a subset of the total number of lights in a room, lighting level can be controlled in a number of discrete steps by virtue of which lights are turned on. To operate well, switchbanks require lights to have overlapping lighting areas.
The more switches used (and hence separately controllable lamps), the finer the control possible.
Dimmer
A Dimmer is an electronic switch that reduces the power flowing through the lamps it controls, which in turn causes a continuously variable change in brightness to be achieved. Note however that the reduction in brightness is not matched in proportion with a reduction in energy consumption since a good proportion of the energy requirement for a filament lamp is just getting it hot enough to begin to glow. As a rough estimate you can say that a 90% reduction in light output may only represent a 50% reduction in power.
Unlike switchbanks dimmers don't require overlapping lighting areas to archive their effect.
Patterns of use
Different people use controllable lighting in different ways. Some will tend to use their lighting at or near to its maximum level most of the time, and then wish to use lower levels occasionally. Some will want to do the reverse, using lower lighting levels everyday, but opting for more brightness on occasion.
Comparison of Energy Efficiency
The pros and cons of switchbank lighting versus dimmer control will vary depending on use patterns. For users who typically use maximum brightness, then neither technique will save much energy, although dimming will have a slight edge since it will tend to extend bulb life a little (hand hence re-lamping costs) by virtue of the slight reduction in maximum brightness that is usually enforced.
For users who routinely use less than full brightness illumination, the energy saving potential of a switchbank is far greater.
This section compares the energy usage requirements for a hypothetical room that is equipped with 300W of filament lighting or the approximate equivalent of 100W of CFL lighting. We will assume the usage pattern in a typical day would be:
1 hour a day of 40w filament equivalent output 5 hours a day of 150w filament equivalent output 1 hour a day of 300w filament equivalent output (i.e. full brightness)
Prices are given for 10p/unit electricity cost.
No Control
Firstly the no control option, for which we will like most typical householders, have to select the wattage best suited to all round use, in this case 150w since that is the most common lighting level required. This is a compromise since we cant have the levels we really want some of the time so comfort and utility not optimal.
7 hours at 150W = 1.05kWh / day = £38 pa = £960 per 25 yr installation life.
If we really must have the full brightness option, then obviously the costs above will double. A more realistic option might be to provide additional table lights etc and keep the main lighting at a more conservative level.
If we opted for 50w of CFL instead:
7 hours at 50W = 0.35kWh / day = £13 pa = £319 per 25 yr installation life.
Switchbank
A total of 300W of filament lamps on switchbank will use: 1 hour at 40w = 0.04kwh + 5 hours at 150w = 0.75kwh + 1 hour at 300w = 0.2kwh
Total usage = 0.99kwh/day = £36 / year or £903 per 25 yr installation life.
CFL Lamps on switchbank will use: 0.015 + 5 x 0.8 + 0.1 kWh = 0.195kWh / day = £7 / year = £177 per 25 yr installation life.
Dimmer
For 300w of halogen on a dimmer: we will assume to get 40w equivilent brightness we run at 50% full current, and for 150w brightness we use 80% of full power
0.5 x 300 + 5 x 0.8 x 300 + 300 = 1.65kwh / day = £60 / year = £1,505 over 25 years
Summary
| Lighting and control | Total cost / year | Total cost / 25 years |
|---|---|---|
| CFL, Full Power No Control | £26 | £638 |
| CFL, Optimal Power, No Control | £13 | £319 |
| CFL, Switch Bank | £7 | £177 |
| CFLs on dimmer | n/a | n/a |
| Filament, Full Power, No Control | £76 | £1,920 |
| Filament, Full Power, Dimmer | £60 | £1,505 |
| Filament, Optimal Power, No Control | £38 | £960 |
| Filament, Switch Bank | £36 | £903 |
From this comparison table several things can be seen:
- The most expensive option is the use of full power lighting with no control.
- Full power lighting with a dimmer is a little cheaper, but not as cheap as using optimal bulb sizing in the first place.
- Switchbanks generally give a lower run cost than control using a dimmer, or opting for no control at all.
- Switchbanks will pay back their extra installation cost many times over if fitted at rewire time.
Reliability
Switchbanks are typically as reliable as any other light switching arrangement (i.e. very), however the need for a larger number of independently controlled lamps will reduce the overall reliability figure a little, since there are more lamps that can fail.
From a light availability point of view however a switchbank provides redundancy, meaning that if one element fails, the system as a whole still continues to work.
A dimmer is a single failure point, and dimmers have variable reliability. The cheaper 250w dimmers seem on the whole to be less robust in this respect. It is generally better to specify a dimmer with greater power handling than you actually require by a reasonable margin (say 50%). A dimmer can operate with a single lamp or cluster of lamps and will hence tend to improve lamp reliability. Many also include a soft start capability that is beneficial in extending halogen lamp life in particular.
Dimmers are rated for their maximum power ratings (and in the case of ones used with LV lamps often a minimum load as well). If dimmers are used over their ratings, early failure is likely.
Compatibility
Standard dimmers are compatible with mains filament bulbs & mains halogens. They are also generally compatible with 12V lighting systems, but a little care mut be taken to pair the correct type of dimmer and transformer or power supply.
Dimmable fluorescent lighting ballasts are available. Dimming these does reduce the energy efficiency of fluorescent lamps to some extent, but nowhere near as much as with filament lamps.
CFLs and dimmers are generally not compatible (special dimmable CFLs are availab;e in some countries, but generally not the UK).
Dimmers have a significant compatibility advantage, which is that they can easily replace a single lightswitch in an existing installation, with no need for any extra wires to be put in. However, depending on how much power and time dimmed lighting is used, retrofitting a switchbank and adding any extra wire needed can work out cheaper, as well as being more energy efficient.
Switching for Switchbanks
Standard faceplates the size of a single light switch can come with upto 3 or 4 switches. Standard double faceplates come with upto 4 or 6 switches. More switches requires a larger faceplate and/or a gridswitch.
It is not difficult to make a standard size faceplate using more switches if this is desired. However 3 and 4 gang switchbanks are normally plenty.
- 2 switches give 3 brightness levels
- 3 switches give anything upto 7 brightness levels
Run and installation Costs
Run cost depends entirely on what lighting is used at what setting for how long, so is a very variable figure. One must also allow for the fact that the heat produced from the lighting will contribute to the heat released into the building. In the winter this may be desirable to an extent and will partially offset heating costs (although using more expensive electric heating in place of cheaper gas or oil). In summer this is counter productive, especially if you have to pay for air-conditioning to vent the unwanted heat.
Lighting run cost is directly proportional to the total power consumed by the light bulbs switched on. When less than the maximum lighting brightness is being used, more cost efficiency is gained with a switch bank than a dimmer.
Fitting a switch bank obviously requires more materials (switches, cable, & luminaries), and may also require significant extra installation time and cost. However this extra install cost can be paid back many times over in the life of the installation.
A switchbank can also be used to mix or choose between filament lighting and CFL. For example, someone that is not keen on CFL might use CFL just for background light, with filament as the main lighting, or use linear fluorescent uplighting with halogen spots for task and feature lighting.
Fitting a Dimmer is usually a simple and low cost option, however the energy saving payback is far less dramatic.
Summary
- Switchbanks are a good idea for new installs, and if used in the intended way, may save many times their cost.
- Dimmers won't save as much money as a switch bank, and if lamp powers are not carefully chosen may even cost more.
- Dimmers are the neat option for multi-bulb chandeliers.
- Dimmers are significantly easier and thus cheaper to retrofit than switchbanks.
- Dimmers give the greatest amount of lighting level control
- Dimmers prevent use of CFL Lamps.