Dimmers & Switchbanks

Dimmers & Switchbanks are both ways to control lighting levels, improving both utility and comfort.

Advantages of Control

Controllable lighting brightness permits

• Bright lighting when its needed for a task
• Relaxed lighting in the evening
• Low lighting at night.

It makes for better comfort and better visibility.

How it affects energy use and running cost depends on which method of control is used.

Summary

• Dimmers make lighting less energy efficient when set to reduced brightness.
• Switchbanks are a good idea for new installs, and on average save many times their cost.
• Dimmers are the neat option for multi-bulb chandeliers.
• Dimmers are significantly easier and thus cheaper to retrofit than switchbanks, but result in higher run cost than a switchbank.
• Dimmers prevent use of CFL Lamps.

Comparison of Energy Efficiency

This section compares each lighting control option for the following desired outcome:

1 hour a day @ 40w filament equivalent output 1 hour a day @ 300w filament equivalent output 1 hour a day @ 500w filament equivalent output

Prices are given for 10p/unit electricity cost.

Switchbank

Filament lamps on switchbank will use: 0.04 + 0.3 + 0.5 kWh = 0.84kWh /day = £30 pa = £766 per 25 yr installation life.

CFL Lamps on switchbank will use: 0.011 + 0.1 + 0.17 kWh = 0.281kWh /day = £10 pa = £255 per 25 yr installation life.

Dimmer

For 500w of halogen on a dimmer: 0.3 + 0.45 + 0.5 = 1.25kWh /day = £45 pa = £1100 per 25yr installation life.

CFLs on dimmer aren't an option.

No Control

Finally the no control option, for which we will, like any typical householder, select the wattage best suited to all round use, in this case 300w. This gives us the real world energy use and run cost, but comfort and utility are not comparable.

3x 0.3 = 0.9kWh /day = £33pa = £820 per 25 yr installation life.

And for 100w of CFL: 3x 0.1kWh = 0.3kWh /day = £11 pa = £270 per 25 yr installation life.

Summary

Energy Use   Option
per 25 yrs
----------   ------
£255         CFL lamps on switchbank
£270         CFL, No Control
£766         Filament lamps on switchbank
£820         Filament, No Control
£1100        Filament on dimmer

From this comparison table 3 things emerge:

1. Switchbanks give lower run cost than either dimmer or no control.
2. Switchbanks pay back their trivial extra cost many times over if fitted at rewire time. Note the above is just for one room of lighting.
3. Perhaps counterintuitively, using a dimmer is the highest energy use option. This is not an error in the maths, looking at the calculation soon reveals why.

Dimmers

Energy Efficiency

A Dimmer reduces the power flowing through a filament lamp by a small amount, causing a large fall off of light output. As an example, 500w of halogen lighting consumes approx 300w while putting out the same amount of light as a 40w filament bulb. Thus dimmers reduce energy efficiency.

Use of dimmers reduces energy use by a fairly limited amount compared to running the bulbs at full brightness, but either replacing the bulbs with more suitably rated ones or use of a switchbank reduces energy use much more.

Use of dimmers is thus an energy wasteful way to reduce light output. The energy wasted is not dissipated in the dimmer, but in the bulbs.

Run Cost

Run cost depends entirely on what lighting is used at what setting for how long, so is a very variable figure.

To get some idea of how dimmers cost money to run, we will compare a 40w filament lamp on full brightness to 500w of halogen lighting dimmed to the same light output.

If your use is different, your figures will be different.

40w Filament Bulb

A 40w filament lamp uses 40w. At 10p/unit that's 0.4p/hr.

With 2 hrs/day of undimmed use, that costs 0.4p x 2 x 365 = £2.92 per year.

Dimmed halogens

500w of halogens dimmed to equivalent the output of a 40w filament bulbs uses 300w.

At 10p/unit that's 3p/hr.

With 2 hrs/day of dimmed use, cost = 3p x 2 x 365 = £21.90 per year.

Over 25 year product life, extra electricity cost use of dimmed high power lighting in place of a more appropriately sized lamp = (21.90 - 2.92) x 25 = £474 This is just for one lamp.

Safety

The ability to use cool running CFLs on switchbanks and inability to use these with dimmers probably accounts for a slight safety difference. In 2001 there were 69,000 house fires in UK. How many of these were caused by hot filament bulbs in fixed fittings is not known, and what percentage of dimmer users would use CFLs if they had a switchbank instead is also not known.

A well known failure mechanism of dimmers is arcing of the potentiometer. This happens more often than switch arcing. Whether this has any significant effect on safety is not known.

What sort of magnitude these differences make is an unknown.

Reliability

A dimmer is a single failure point, and dimmers have variable reliability. 250w dimmers seem on the whole to be less robust in this respect.

Dimmers have limited power ratings, so extension of the lighting circuit or fitting of higher power bulbs may cause overload.

If dimmers are used over their ratings, early failure is likely.

Compatibility

Standard dimmers are compatible with mains filament bulbs & mains halogens.

Other types of lighting require a ballast or power supply that is specifically designed to be dimmable, plus a dimmer designed to work with them.

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 not compatible.

Dimmers have one significant advantage, which is that they can easily replace a single lightswitch in an existing installation, with no need for any extra wire 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.

Switchbanks

A Switchbank is a bank of 2 or more switches. If each switch controls 1 or some of the lights in a room, lighting level can be controlled by which lights are turned on.

The more switches are used, the finer the control possible.

Sizes

Standard faceplates the size of a single lightswitch come with upto 3 switches. Standard double faceplates come with upto 4 switches. More switches requires a larger faceplate.

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

Energy Efficiency

All lights used operate at full normal efficiency. Lighting controlled by a switchbank is thus much more energy efficient than when using dimmers (at below full brightness setting).

Greater energy efficiency means less heat, and this is welcome on hot summer days.

Switchbanks also leave open the choice of filament, fluorescent or CFL, unlike dimmers. In some cases this will also produce significant energy efficiency improvement.

Run Cost

Lighting run cost is directly proportional to the total power of light bulbs switched on. When less than the maximum lighting brightness is being used, this is a much lower cost option than a dimmer.

Fitting a 2 gang switch in place of a single switch can cost an extra £1, and the extra cable might cost £1, or more with a long run. Typically this minimal extra install cost is 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.

Safety

A switchbank enables very low power night lighting to be used without the need for a plug in light. Total run cost for a 3w cfl lamp used 8 hours a night is under £2 per year.

Nightlights can of course be used in plug-in lights without a switchbank, but a fixed fitting is neater and a bit safer with kids about.

A nightlight reduces night time accidents, and is good for kids afraid of the dark.

A switchbank also enables use of CFLs, which are a lower fire risk than hot filament lamps.

Reliability

Switches are an order of magnitude more reliable than dimmers.

A switchbank is a redundant array from a reliability viewpoint, meaning that if one element fails, the system as a whole still continues to work.

These 2 points make a switchbank very much more reliable than a dimmer.

Compatibility

Switchbanks are compatible with all types of domestic lighting.

Switchbanks have a power rating equal to that of nearly all domestic lighting circuits, so the need to limit lightbulb power or upgrade the switches does not occur.

To operate well, switchbanks require lights to have overlapping lighting areas. Dimmers don't require this.

Switchbanks have one compatiblity limitation, which is that replacing a single switch in an existing installation will often require an extra wire to be run from switchbank to light fitting. This is not always the case, but frequently is. While trivial at rewire time, it is often not so trivial with retrofit, and limits their use, despite the run cost advantage.

Dimmable CFLs

These exist but are very hard to find in the UK. They typically have 2 or 3 brightness levels rather than being continuously dimmable.

See Also

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