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Engineering Guide

How to Spec
Solar Lighting

The stamp on the drawing says approved. The parking lot at 2 a.m. in January says otherwise. Solar lighting is an energy system wearing the shape of a lighting fixture, and the standard submittal review was built for a technology that never runs out of power.

Jump to spec languageSee the energy math
From one submittal that passed review
945 Wh
Nightly energy deficit on a submittal that passed review
37 W
Actual sustained load on a fixture modeled at 120 W
50%
Harvest variance between poles on the same project

The people reviewing solar lighting submittals were trained on a technology that connects to the grid. The criteria are clear, the load is fixed, and performance does not change between Tuesday and Saturday based on how many clouds passed over the site last week.

Solar is not that. This is not a criticism of engineers. It is an honest description of a gap the industry created by selling solar lighting as a product swap instead of what it actually is. Nobody taught the review process how to read an energy budget. Nobody taught the specifier which questions expose a fabricated runtime claim. And the vendor, who knows the difference, is not volunteering the lesson.

This guide covers the five places a solar submittal goes quiet, the arithmetic that exposes an inflated runtime claim in about ten minutes, and specification language you can lift directly into your next package.

A Complete Package That Answers Nothing

Both columns describe the same submittal. The left column is what the review template asks for. The right column is what determines whether the system works in February.

In the submittal

  • Fixture cut sheet
  • Photometric plan and calculation zones
  • BUG rating table
  • Pole and foundation schedule
  • Peak lumen output
  • Listings and warranty card

Never in the submittal

  • Worst-month energy budget
  • Panel azimuth at each pole location
  • Battery state-of-charge curve across a full night
  • Sustained operating wattage behind the photometrics
  • Dimming floor tied to a criteria value
  • Definition of what the monitoring actually reports

Five Places the Submittal Goes Quiet

Each of these is standard practice, not vendor misconduct. That is exactly why the review process misses them.

The submittal looks complete because it answers the wrong questions

A solar lighting submittal arrives with everything a grid-tied review expects: cut sheet, photometric plan, BUG rating, pole schedule, lumen output, warranty card. The reviewer checks the boxes that have always needed checking, the footcandle averages comply, the uniformity ratios pass, the fixture is listed. Approved.

What is missing is every number about energy. No harvest calculation. No battery sizing verification. No runtime curve. No statement of what the system delivers at hour eight of a January night. The package satisfies the review template and says nothing about whether the system survives the winter.

Ask for the energy budget before you open the photometric plan.

The photometric plan is one perfect moment that will never occur

The plan shows a fully charged fixture at full rated output, aimed perfectly, in software with no weather, no battery state, no temperature, and no time of night. It is a static calculation of a dynamic system frozen at its theoretical best.

The same poles at hour six of a January night look nothing like the plan. Lithium iron phosphate does not deliver rated capacity at minus 15. The controller engages its dimming floor at midnight to protect the pack. None of that appears on a drawing calculated at full power and accepted at face value.

Ask which wattage the calculation was run at, and how long the system holds it.

The dimming floor is a battery decision wearing a lighting badge

A 120 W all-in-one rated at 22,200 lumens that dims to 30 percent is operating at roughly 37 W for most of the night. The photometric plan was almost certainly modeled at 120 W.

Nobody checks whether 37 W at that pole spacing and mounting height still delivers the specified footcandles. It does not. The floor was set at the factory to keep the battery alive, and it arrives on the approved drawing as a compliance checkmark.

Ask what illuminance the dimming floor is tied to. If the answer is a percentage, there is no criteria behind it.

The panel faces wherever the pole faces, and no document shows it

In an all-in-one fixture the panel and the LED are one rigid assembly. Poles are placed to serve the photometric layout, so fixtures along different drive aisles face east, west, north, and every azimuth in between. Each one harvests a different amount of energy every day.

The result is not just less energy, it is inconsistent energy pole by pole. Each fixture hits its protection threshold at a different hour. One dims at midnight, the next holds until 2 a.m., the one after that cuts out by 11 p.m. The lot becomes a checkerboard that shifts with the weather.

Ask for the azimuth of every pole, not a project average.

"Remote monitoring" usually means "the light is on"

Specifiers read remote monitoring as a performance assurance feature. On most budget systems it reports on or off, sometimes a battery percentage estimated from a voltage curve that drifts with temperature and age, sometimes a dimming schedule you can edit if the app works and the firmware is current.

What it almost never reports is whether the system is meeting the lighting criteria it was specified to maintain. A system that dims itself into non-compliance and reports "operational" is a liability dressed up as a dashboard.

Ask for a sample report. If it cannot show delivered output against design output, it is not performance monitoring.

The Energy Math Nobody Ran

A real submittal: a large site, more than eighty poles, single and dual-head all-in-one fixtures rated at 120 W, specified for a 14-hour runtime. The photometric plan complied in every calculation zone. Here is the arithmetic the package did not include.

Energy required120 W for 14 hours, before losses
1,680 Wh
Battery usable921.6 Wh nominal at 80% depth of discharge
737 Wh
Winter harvest, facing southAt 3.5 peak sun hours
595 Wh
Winter harvest, 60° off southThe north-south drive aisle case
300 Wh
945 Wh+
Nightly deficit at rated power

The battery cannot deliver half of what a full-output night requires, and the panel cannot replace what the battery gives up. Even the upgraded 48 Ah variant falls well short. The submittal said nothing about any of this, the review process had no mechanism to catch it, and the stamp went on the drawing.

The fine print qualifies the 14-hour claim with “alternate working modes.” That phrase is the specification admitting the fixture cannot run at 120 W for 14 hours. In practice the system survives the night by dimming aggressively, and 22,200 lumens is a peak rating rather than a sustained operating condition. When the photometric design was built around a target footcandle level, the difference between peak and sustained output is not a footnote. It is the entire basis of whether the design works.

What the Drawing Shows, and What the Night Does

The photometric plan is the flat line. The system is the curve underneath it. Every hour between them is a compliance gap nobody signed off on.

100%30%0%6p8p10p12a2a4a6a8aModeled on the photometric plan, 120 WDelivered after the dimming floor engages, 37 WBattery protection engagesThe compliance gapIllustrative profile for a 120 W all-in-one fixture across a 14-hour winter night in a northern climate.

Scroll the chart sideways to read the full night.

Harvest Penalty by Panel Azimuth

Approximate daily harvest loss relative to true south for a fixed panel in a northern-hemisphere layout. The variance between a well-oriented pole and a poorly oriented one on the same project can exceed 50 percent.

0°
True south
no loss

Design basis. Rarely achieved on more than a fraction of poles.

30°
Off south
-13% harvest

Common on angled drive aisles and curved roadways.

45°
Off south
-30% harvest

Typical corner and perimeter poles.

60°
Off south
-50% harvest

A panel running parallel to a north-south drive aisle.

The IES file used for the photometric calculation assumes one fixture, fully charged, perfectly aimed, on the best night of the year. The pole-by-pole azimuth reality of a real parking lot is invisible to the review process from start to finish.

Copy into your spec

Specification Language You Can Use

The fix is not to blame the reviewer. The fix is to change what the submittal is required to contain, so the gap between rated and real performance is visible before the stamp goes on the drawing.

2.6.A

Worst-month energy budget

Submit a pole-by-pole energy budget for the worst solar month at the project latitude, using site-specific peak sun hours. Derate for temperature, soiling, wiring losses, battery round-trip efficiency, and driver efficiency. State each derate factor and its source.

2.6.B

Panel azimuth and harvest penalty

Submit the panel azimuth at every pole location in the approved layout, with the harvest penalty at each location relative to true south. A single project-wide harvest figure is not acceptable.

2.6.C

Battery state-of-charge curve

Submit a battery state-of-charge curve across a full worst-month night at the project latitude, demonstrating the system stays above its low-voltage protection threshold through the end of the operating window, at the worst-oriented pole on the project.

2.6.D

Sustained output, not peak output

Photometric calculations shall be performed at the sustained operating wattage the control profile actually maintains through the operating window, not at peak rated output. Submit the dimming schedule and the wattage at each step.

2.6.E

Dimming floor tied to criteria

The minimum dimming level shall be tied to a documented illuminance criteria value. The system shall not dim below the level required to maintain specified minimum footcandles and uniformity.

2.6.F

Monitoring that reports compliance

Remote monitoring shall report delivered output as a percentage of design output, battery state of charge, and daily harvest per site, with alerting when any site falls below the specified criteria. On/off status alone does not satisfy this requirement.

Three Questions Before You Accept the Submittal

Put these to any solar lighting vendor. If they cannot be answered with documented numbers, the proposal is built on bench geometry and best-case assumptions rather than on what will actually be installed and operating on this site.

01

What is the panel azimuth at each pole location in this layout, and what is the harvest penalty at each one relative to true south?

A project-wide average hides the poles that fail first.

02

What is the pole-by-pole energy budget across this project using actual winter peak sun hours and actual panel orientation?

This is the document that makes the gap visible before the stamp goes on.

03

Can you provide a battery state-of-charge curve across a full winter night at the project latitude showing the system stays above its protection threshold until the end of the operating window?

If the curve crosses the threshold, the lot goes dark before sunrise.

How Clear Blue Answers Them

Sized against 30 years of site weather

Systems are modeled at the project GPS coordinates using historical irradiance for the worst month, not a national average or a bench figure.

Energy managed against a criteria target

Illumience manages the nightly energy budget so output is held to the specified lighting profile, rather than letting a factory dimming floor decide the light level.

Monitoring that reports performance

Delivered output, state of charge, and daily harvest are tracked per site with alerting, so underperformance surfaces before the client calls about a dark lot.

Source and credit

This guide is adapted, with permission, from “Nobody Taught the Engineer How to Read a Solar Submittal” by Piotr Mikus, MIES, a roadway lighting designer and solar lighting specifier who teaches solar submittal review as continuing education for licensed engineers. The original appears in the After Dark series at solarlightingnightshift.com. Views expressed there are the author’s own.

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Have a Solar Lighting Submittal on Your Desk?

Send us the layout and the fixture data. Clear Blue engineers will run the worst-month energy budget pole by pole and show you where the numbers land.

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