PIR vs Time Dimming for Solar Street Lights — Energy, Reliability & Procurement Rules

Dimming is the hidden variable that breaks quote comparisons. Two lights with the “same wattage” can have totally different real energy use depending on the dimming profile. This guide explains PIR vs time-based dimming in a procurement-ready way: how to compare, what risks to watch, what to record during commissioning, and how to write RFQ clauses so bids become apples-to-apples.

Where This Guide Fits

TL;DR (Decision rules)

  • Time dimming is predictable. It is easier to model (Wh/night) and easier to compare quotes fairly.
  • PIR dimming can save energy in low-traffic areas, but requires clear assumptions (base vs boost time) and reliable sensor placement/commissioning.
  • Procurement rule: any bid without a declared dimming profile (or PIR assumptions) is Not Comparable.
  • Commissioning rule: dimming mode and settings must be recorded; otherwise warranty and troubleshooting become un-auditable.

Key internal references:

Definitions (procurement-friendly)

Time dimming (schedule-based)

The light follows a fixed schedule after dusk (e.g., 100% for 2 hours, 50% for 6 hours, 30% until dawn). It is predictable and easy to model in the energy budget sheet.

PIR dimming (motion-based)

The light runs at a base level (low power) and boosts to a higher level when motion is detected. Energy depends on traffic: how often motion triggers and how long boost stays on.

Why this matters for solar sizing (Wavg is everything)

Solar sizing is driven by average power (Wavg) over the night. Wavg drives:

  • Daily energy use (Wh/night)
  • Battery required usable energy
  • PV required charging yield (especially worst-month)

Use these sizing pages as the “math backbone”:

Time dimming: strengths, risks, and where it fits

Strengths

  • Predictable energy: easiest to calculate and audit (Wh/night).
  • Quote comparability: suppliers can be forced to use the same schedule.
  • Commissioning simplicity: fewer variables than PIR traffic assumptions.

Risks

  • If the schedule is not documented, site teams cannot confirm Wavg assumptions.
  • “Too aggressive” dimming can create user complaints if brightness drops too early.

Best-fit use cases

  • Main roads where predictable performance is required
  • Projects where bid comparability and auditability are critical

PIR dimming: strengths, risks, and where it fits

Strengths

  • Potential energy savings in low-traffic zones (parks, pathways, rural roads).
  • Can improve perceived “responsiveness” when someone approaches.

Risks (what breaks projects)

  • Hidden assumption risk: savings depend on motion frequency and boost duration.
  • Sensor placement risk: wrong mounting/angle can miss motion or false-trigger.
  • Commissioning complexity: sensitivity and boost settings must be recorded.
  • Comparability risk: two suppliers can claim the same autonomy with totally different PIR assumptions.

Best-fit use cases

  • Low-traffic areas where energy saving is a primary goal
  • Sites that can accept variable energy behavior (depends on motion)

Apples-to-apples comparison method (mandatory for procurement)

To compare quotes fairly, you must force suppliers to use a declared, comparable dimming model.

  • Define one schedule (Scenario A) and require all suppliers to quote against it.
  • This creates clean Wh/night comparability across bids.

Option B (if you must use PIR): require a PIR assumption disclosure

Require the supplier to disclose these PIR assumptions in the quote:

  • Base level (percentage or watts)
  • Boost level (percentage or watts)
  • Boost duration (seconds/minutes)
  • Assumed motion events per hour (or per night) used to model Wavg
  • Sensor range and mounting orientation assumption

Procurement rule: if PIR assumptions are not disclosed, the quote is Not Comparable.

Use this page to enforce bid comparability:

Commissioning and acceptance (dimming must be recorded)

Commissioning record requirements

  • Dimming mode used (time / PIR / hybrid)
  • Schedule steps (time dimming) OR base/boost/sensitivity settings (PIR)
  • Runtime intent (dusk-to-dawn or fixed hours)
  • Any later setting change must be logged (date + reason)

Use the operational template here:

Failure patterns linked to dimming (field diagnosis)

  • Early shutoff after cloudy weeks: Wavg higher than assumed OR worst-month sizing deficit.
  • Complaints about “too dark”: schedule too aggressive OR PIR base level too low.
  • Random behavior: loose connections, ingress, or configuration not standardized.

Reference:

PASS/FAIL procurement rules

Mark a bid FAIL (Not Comparable) if it lacks:

  • Dimming mode disclosure (time / PIR / hybrid)
  • Time schedule steps OR PIR base/boost/boost-time assumptions
  • Energy budget sheet (Wavg, runtime hours, Wh/night)
  • Commissioning record template (settings must be auditable)

Copy/Paste RFQ Clauses (PIR vs Time Dimming)

Clause 1 — Dimming profile disclosure (mandatory)

Supplier shall disclose the dimming method (time/PIR/hybrid) and the exact dimming profile used for sizing and quotation. Quotes without a declared dimming profile are not comparable.

Clause 2 — Energy budget sheet (mandatory)

Supplier shall provide an energy budget sheet including Wavg, runtime hours, and Wh/night based on the declared dimming profile.

Clause 3 — PIR assumptions disclosure (if PIR is used)

If PIR dimming is used, supplier shall disclose base level, boost level, boost duration, and assumed motion event frequency used to model average power (Wavg). Supplier shall also disclose PIR sensor range and mounting orientation assumptions.

Clause 4 — Commissioning record (mandatory)

Supplier/contractor shall provide a commissioning record template and deliver commissioning settings (mode + schedule or PIR settings) as part of handover. Any settings change shall be logged with date and reason.

Request Dimming Profile & Energy Budget Templates

Get the editable templates used to standardize dimming assumptions and prevent non-comparable bids:

  • Time-dimming schedule template (Scenario A/B/C)
  • PIR assumptions disclosure sheet (base/boost/boost-time/events)
  • Energy budget sheet template (Wavg, hours, Wh/night)
  • Commissioning record template (mode + settings)

Want a quick engineering sanity check?

Share your road class, pole height, target lux/uniformity, and worst-month PSH assumptions.

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Next Steps (Keep Your Project Tender-Ready)

Ensure your solar street light project meets all procurement and engineering standards.

Templates & Evidence

Shortlist a System Type

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FAQ

Which is better: PIR or time dimming?

Time dimming is better when predictability and bid comparability matter (roads and critical projects). PIR can save energy in low-traffic areas if assumptions are disclosed and commissioning is done properly.

Why do PIR quotes often look “too good”?

Because savings depend on assumed motion frequency and boost duration. Without disclosed assumptions, the energy model cannot be audited, so the quote is not apples-to-apples.

What must be recorded at commissioning?

The dimming mode and exact settings (schedule steps for time dimming, or base/boost/sensitivity/boost-time for PIR) plus any later changes with date and reason.