Where motion-sensor lighting really saves energy

A motion sensor only lights a space when someone is there, but the benefit varies hugely by room. How sensors work, where they suit and the usual installation mistakes.

The idea behind a motion sensor is simple: the light comes on when someone arrives and goes off a while after they leave. Nobody can forget to switch off, and empty rooms stay dark. In practice, though, a sensor brings real savings and convenience in some places, while in others it becomes an irritating gadget that saves very little. Knowing the difference helps you put it in the right spot.

How a sensor “sees” movement

Two kinds of sensor are common in homes and small premises.

A passive infrared (PIR) sensor detects the change a warm human body makes against the background. It is best at catching movement across its field of view, and can be slightly slow to notice someone walking straight towards it. A PIR sensor cannot see through glass and does not react to movement behind a wall.

A microwave (radar) sensor emits a high-frequency signal and reacts to changes in the reflection. It picks up smaller movements and can see through some thin partitions. That is as much a risk as an advantage: the sensor may switch the light on because of movement in the next room or out in the street.

Both types usually have three settings: sensitivity, how long the light stays on, and a light-level threshold so that the sensor does not switch on when the space is already bright enough in daytime.

Where the saving is real

The places where sensors pay off share one trait: people pass through briefly and the light is often forgotten.

• Stairwells and landings in apartment blocks. Instead of burning all night, the light runs only while someone is passing.
• Storerooms, garages and basements. People walking in with full hands do not hunt for the switch, and nobody forgets to turn it off.
• Office toilets, archives and utility rooms. These lights otherwise burn empty for much of the day.
• Yard entrances, gates and parking spaces. Here the sensor serves both saving and security.
• Hallways and night paths at home. Low light that comes on automatically at night reduces the risk of falls.

Where sensors annoy people

Sensors work poorly in rooms where you sit still for long periods. Watching TV in the living room, working at a computer in a study or reading in bed, the light can suddenly go out and leave you waving your arms. In these rooms an ordinary switch, a dimmer or at most an “absence” mode works better: you switch the light on yourself, and the sensor only turns it off after you leave.

The saving in living spaces is also relatively small, because the light is on when people are there anyway and is often switched off when they leave.

Choosing the time delay

The most common mistake is setting the delay too short or too long. Too short, and the light goes off while someone is still in the room, then comes back on again and again. Too long, and the sensor is little better than a switch: a one-minute trip keeps the light on for a quarter of an hour.

A practical approach: for stairs and hallways, a short delay that comfortably covers the walk; for storerooms and toilets, a little longer than people usually spend there. Watch how it behaves in the first week and adjust if needed.

Compatibility with LED bulbs

Pairing a sensor with LED bulbs sometimes causes two issues. First, two-wire sensors without a neutral draw their own power by passing a small current through the lamp. That went unnoticed with incandescent bulbs, but an LED may glow faintly or blink when it should be off. The fix is a sensor with a neutral connection, or the load element the sensor manufacturer recommends.

Second, the sensor’s relay contacts can be stressed by the brief inrush current when LED drivers start, especially with many lamps on one sensor. Check whether the sensor’s data sheet gives a separate limit for LED loads.

Installing a sensor means working on the electrical circuit, so have a qualified electrician do it.

Estimating the saving

The benefit depends on how long the light used to burn with nobody there. If a stairwell light used to run 10–12 hours a night, a sensor can cut that to less than an hour of real use. If a storeroom light was already switched off carefully, the saving may take a very long time to cover the sensor’s cost. Watching for a few days how long the light burns empty gives the most objective basis for the decision.

Remember that the sensor itself draws a little power on standby. It is usually tiny next to the saving, but worth counting in a rarely used space.

The Zəfər İşığı catalogue includes E27 LED bulbs in several wattages that can be used with sensors in stairwell, storeroom and outdoor fittings. Questions about compatibility with a particular sensor can be sent through Zaferoğlu Elektrik’s enquiry form.