Microwave vs PIR: Choosing Sensor Lighting That Actually Pays Back

Sensor-controlled lighting is the rare energy upgrade with a payback under two years — when the sensor technology matches the building. Spec the wrong detection principle for a high-rack warehouse and you get flickering aisles, complaints, and a retrofit that gets switched off in month three. Here is how the technologies differ and how to build an ROI number a CFO signs off.

PIR vs microwave vs daylight sensing

Technology	How it detects	Strengths	Limits	Best fit
PIR (passive infrared)	Warm-body heat movement across zones	Cheap, reliable, low false triggers	Needs line of sight; range drops at height; blinds behind racking	Offices, corridors, low halls (≤ 4–5 m)
Microwave (Doppler)	Radio echo shift from any motion	Sees through plastic, glass and thin partitions; works at 8–14 m mounting	Detects motion outside the room if sensitivity unadjusted; slightly higher cost	Warehouses, high-bay, cold rooms
Daylight harvesting	Photocell dims against window light	Captures free daylight automatically	Needs commissioning per zone	Perimeter aisles, skylit halls
High-frequency hybrid	Microwave + daylight cell in one unit	One device, full control	Commissioning still required	New warehouse fit-outs

The height rule is the one buyers forget: a PIR sensor mounted at 10 m over a warehouse aisle watches a tiny floor footprint and cools down slowly. Microwave at the same height covers long aisles and reacts to a forklift a full bay away.

The physics of the savings

Savings come from what happens when nobody is there. The two strategies:

• Off/on switching. Full cut when vacant. Maximum savings, but harsh in warehouses where a picker entering a dark aisle wants light now — and restart wear worries buyers used to HID lamps (with LED, instant restrike is a non-issue: no warm-up, no lamp wear from cycling).
• Dim-to-low. Drop to 10–20% output when vacant, full on at detection. The typical compromise for 24/7 logistics: safety and camera coverage maintained, 60–80% of the vacant-period energy still saved.

Real occupancy data from logistics facilities puts aisles at 30–50% occupied during operating hours, and near zero overnight. That vacant share is the entire business case.

A payback you can defend

Worked example — a 100-fixture warehouse, UFO high bays at 150 W, 12 operating hours/day, electricity at €0.15/kWh, microwave dim-to-low controls:

Line	Baseline (no sensors)	With dim-to-low sensors
Installed load	100 × 150 W = 15 kW	100 × 150 W + 1 W sensor ≈ 15.1 kW
Annual burn @ 12 h/day	15 kW × 4,380 h = 65,700 kWh	Occupied 40% at full + 60% at 15% ≈ 31,800 kWh
Annual energy cost	€9,855	€4,770
Annual saving	—	≈ €5,085
Sensor control adder (typical)	—	€12–18 per fixture installed
Simple payback	—	Under 5 months

Even at half the occupancy savings — say the facility runs three shifts — payback stays inside twelve months. The sensitivity: electricity tariff and actual occupancy. Ask your supplier for a DIALux + occupancy simulation against your shift pattern rather than a brochure percentage.

Spec pitfalls that kill sensor projects

• Mounting height vs sensor rating. Verify the sensor is rated for the actual mounting height; a 6 m-rated PIR at 12 m gives dead zones under the fixture.
• Metal racking multipath. Microwave reflects off racking and steel cladding. Choose fixtures with adjustable sensitivity and hold-off time, and commission one aisle before the whole hall.
• Control wiring topology. Decide early between standalone (each fixture its own sensor), master-slave rows (one sensor drives a batten run), or a DALI/BLE mesh for scene control. Retrofitting topology later means rewiring.
• Cold storage. Standard sensors and drivers drift below −20 ℃. Specify cold-rated components explicitly — we validate operation at −40 ℃ on request.

Frequently asked

Do sensors shorten LED lifetime?

No — unlike HID, LEDs do not suffer from switching cycles. A quality driver rated for frequent on/off exceeds the fixture's own service life. Dimming to 10–20% can actually extend driver electrolytic-capacitor life by reducing heat.

Can one sensor control a whole row of battens?

Yes — master-slave wiring lets one sensor unit switch or dim a linked run, cutting sensor cost per meter. For warehouses we usually recommend per-fixture microwave sensors on high bays and master-slave on linear batten rows.

What happens when nobody moves for a long time — will the lights drop during a meeting or shift?

Settings solve this: hold-off time (how long after last detection), and for offices a presence-sensitivity mode that detects small movements like typing. Commissioning is where good projects separate from nuisance-flicker projects.

Is the payback still real with cheap electricity?

Scale it: halve the tariff and the example above still pays back in about ten months. The other driver is regulation — the EU EPBD and local codes increasingly mandate controls in new and renovated commercial buildings, so the baseline comparison itself is disappearing.