Dimmed PIR Lights

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Outdoor security lights are available that operate the light bulb in a dim mode most of the time, and switch to full brightness when human motion is detected.

On the face of it these sound like a good idea, but as with many things in life, all is not as it first seems.

  • NOTE * this article was written about filament bulbs. Times have moved on, dimming LEDs does not cause significant loss of efficiency, so this problem does not occur with LEDs.

The problem

The problem is gross energy inefficiency, wasting thousands of pounds on running costs.

How Come?

1. Dimming lightbulbs means using filament lamps, as dimmable CFLs are rare.

2. Dimmed filament lamps are highly inefficient light sources. On the scale of A-G for energy efficiency, dimmed filament bulbs are off the scale.

A dimmed 500w halogen lamp was tested and found to consume 300w when giving out the equivalent amount of light as a 40w GLS (filament) bulb.

For comparison, a 9w CFL gives a similar light level output, yet consumes only 3% as much energy. That's a 97% saving on energy use and cost.

3. So using a 100w bulb on one of these means in dim mode it wll consume in the region of 60w all night long. An always on CFL giving a similar output would consume around a tenth that energy and money.

4. But the bigger problem occurs with 500w halogen versions of these. A halogen lamp dimmed to give out the light of a 40w bulb consumes 300 watts. If we take winter nights as 16 hrs and summer as 12 hrs, and electicity cost of 10p per unit, we get an annual run cost of:

14*365*.3*10 = £153 per year for the dimmed lighting alone.

A 9w CFL on all night would give similar light output, at a cost of:

14*365*.009*10 = £4:60 per year.

The dimmed fitting costs an extra £148.70 per annum in wasted energy.

Over a 25 year product life, this is £3,717.

That's right, making one little bad choice will cost you over 3 grand in extra costs.



  • If you want a PIR light, a 100w bulb on a PIR is a good choice for most houses.
  • 150w halogen is useful where higher brightness is wanted.
  • To light larger areas, position the lighting higher up
  • 500w halogen is heavy overkill for the average house, and better suited to large outdoor areas. To be effective at lighting a large area it needs to be mounted high up.

If you want low level lighting all night long, a CFL can provide it at minimal cost and with minimal energy consuption. Dimmed PIR fittings should not be used for this.

See Also