Dimmed PIR Lights

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.

The problem

The problem is gross running costs and terrible energy efficiency.

How Come?

1. Dimming lightbulbs means using filament lamps, as nearly all CFLs are not dimmable.

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 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 giving 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=£153:30 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 £3,717:50

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

The Moral of the Story

• Don't run filament bulbs dimmed.
• Don't use 500w halogens where 100w bulbs would be more suitable
• Be aware of what you're buying, or you'll pay the price

Solutions

If you want a PIR light, a 100w bulb on a PIR is the best choice for most houses. 500w halogen is real overkill for the average house.

If for some reason you need low level lighting all night long, a CFL will provide it at minimal cost and with minimal energy consuption. Don't use a dimmed PIR fitting for this.

See Also

Rewiring Tips

Lighting Glossary

keywords: Dimmed PIR Lights