Solar Street Light Lumen Depreciation: L70/L80 & TM-21 Decoded

“High lumen” claims collapse over time if heat is not managed. In solar street lights, thermal design and lumen depreciation determine two outcomes: (1) whether road lighting stays acceptable after years, and (2) whether the energy budget stays realistic (because pushing more current for more lumens usually increases power and heat). This guide shows what to request, what evidence matters, and how to write tender-ready clauses so quotes become comparable.

Where This Guide Fits

TL;DR (What to do in procurement)

  • Do not buy “LED chip lumens”. Require fixture (delivered) lumens at a declared ambient temperature.
  • Heat drives depreciation. Lumen maintenance depends on LED junction temperature and driver temperature behavior.
  • Evidence rule: if a supplier claims long lifetime, ask for LM-80 / TM-21 evidence (LED level) plus a clear fixture-level thermal statement.
  • Apples-to-apples: quotes must declare CCT/CRI, optic/IES, and delivered lumens under the same assumptions.

Key internal references:

Definitions (procurement-friendly)

Thermal management

How the luminaire moves heat away from LEDs and driver electronics. Poor thermal design increases temperatures, accelerating lumen depreciation and component failures.

Lumen depreciation (lumen maintenance)

The gradual reduction of light output over time. A key term is L70: the time when light output declines to 70% of initial output (at specified conditions).

LED-level vs fixture-level claims

  • LED-level: a test/projection for the LED package (often supported by LM-80/TM-21).
  • Fixture-level: what the installed luminaire delivers after optics, thermal environment, and driver behavior. This is what matters for roads.

Why solar street lights are sensitive to thermal design

  • Higher drive current to chase lumens increases heat and can reduce lifetime.
  • Sealed housings (for IP protection) can trap heat if design is not balanced.
  • Hot climates raise ambient temperature, reducing thermal margin.
  • Thermal + moisture is a common failure amplifier (drivers, connectors, lenses).

Use these pages to connect thermal design to real outcomes:

What you should specify (so performance stays acceptable after years)

1) Delivered lumens (not marketing lumens)

  • Require fixture delivered lumens for the quoted configuration.
  • Require the supplier to declare the conditions used (ambient temperature assumption).
  • Require the same basis across bidders.

2) Maintenance factor / end-of-life intent

If your design requires a minimum light level on the road, you should not size only by “day-1 lumens.” Require a declared end-of-life intent (for example, maintaining acceptable performance after years under the declared thermal environment).

3) CCT/CRI and optic consistency (comparability)

Different CCT/CRI and optics can change perceived brightness and distribution. Lock these in the RFQ so quotes remain comparable.

What evidence to request (so “50,000 hours” is not a slogan)

A) LED package evidence (where available)

  • LED brand and model (not just “high quality LED”).
  • LM-80 report for the LED package (test data basis).
  • TM-21 projection if they provide lifetime projections (projection method).

B) Fixture-level thermal statement (the practical minimum)

Ask the supplier to provide a clear fixture-level statement that ties the configuration to heat behavior, for example:

  • Declared ambient temperature rating (e.g., typical project ambient range)
  • How the fixture manages heat (heat sink design, conduction path)
  • Driver placement and thermal protection behavior (over-temp derating or protection)

Procurement reality: even if LED package evidence exists, the fixture can still fail early if driver thermal protection and sealing are poor.

How thermal design affects your energy budget (Wavg and sizing)

Thermal design influences how hard the system must drive the LEDs to meet brightness targets. More drive current usually means higher power and higher Wavg.

  • Higher Wavg increases Wh/night demand.
  • That increases battery usable energy requirements and PV charging pressure.

Use these pages to keep sizing honest:

Field acceptance & commissioning (what to record so issues are diagnosable)

  • Configuration match: confirm model/optic matches the quoted IES/simulation package.
  • Dimming record: confirm dimming mode and profile are recorded (affects heat and Wavg).
  • Evidence pack: keep serial-to-location mapping and installation photos for warranty enforceability.

Operational reference:

Common red flags (thermal + lumen claim traps)

  • They quote “LED chip lumens” but refuse to state delivered lumens.
  • They claim very long lifetime but cannot identify LED model or provide any supporting evidence.
  • They change optics or CCT between quotes (not apples-to-apples).
  • No commissioning record template (settings drift makes failures un-auditable).

PASS/FAIL procurement rules

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

  • Declared delivered lumens and the basis (configuration and conditions)
  • Declared CCT/CRI and optics basis (IES/simulation package alignment)
  • Dimming profile disclosure (affects Wavg and heat)
  • Warranty terms that reference evidence (serial mapping, commissioning record)

Use this for comparability enforcement:

Copy/Paste RFQ Clauses (Thermal & Lumen Depreciation)

Clause 1 — Delivered lumens disclosure (mandatory)

Supplier shall provide fixture delivered lumens for the quoted configuration (including optics and CCT). “LED chip lumens” or marketing lumens are not acceptable for bid comparison.

Clause 2 — Lumen maintenance evidence (mandatory for lifetime claims)

If supplier claims long lifetime or lumen maintenance (e.g., L70), supplier shall identify the LED model used and provide supporting evidence such as LM-80 data and TM-21 projection (where available) or equivalent documentation for the LED package.

Clause 3 — Thermal rating statement (mandatory)

Supplier shall provide a fixture-level thermal statement including ambient temperature rating assumptions and thermal protection behavior (driver/LED over-temperature behavior) for the quoted configuration.

Clause 4 — Comparability lock

Supplier shall declare CCT/CRI, optics (IES file reference), and dimming profile used for sizing and quotation. Quotes without these disclosures are not comparable.

Request Lumen Maintenance & Evidence Checklist

Get the editable checklist used to eliminate “lifetime slogans” and standardize bid evidence:

  • Delivered lumens declaration template
  • LED evidence request checklist (LED model + LM-80/TM-21 where available)
  • Thermal rating statement template
  • Bid PASS/FAIL comparability screen

Want a quick engineering sanity check?

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

Contact Inlux Solar →

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

What is the difference between LED L70 and fixture L70?

LED L70 refers to the LED package under test/projection conditions. Fixture performance depends on the complete luminaire: optics losses, driver behavior, sealing, and real operating temperature. Procurement should prioritize delivered lumens and auditable evidence.

Why does heat accelerate lumen depreciation?

Higher temperatures generally accelerate aging mechanisms in LEDs and drivers. If the thermal path is weak, junction and driver temperatures rise, increasing lumen depreciation and failure risk.

How can I make thermal/lifetime claims comparable across bids?

Require delivered lumens under declared conditions, lock CCT/optics/dimming assumptions, and request evidence for lifetime claims (LED model identification and supporting documentation) plus a fixture-level thermal statement.