LED retrofits are visible, measurable, and often supported by fixture schedules and utility analyses. That makes them attractive screening candidates. It does not mean a wattage reduction by itself establishes a Section 179D deduction under current law.
The ordinary route now uses a whole-building modeled annual energy-cost comparison. Older partial-lighting rules should not be imported into current projects, and historical projects must be evaluated under the law applicable to their period.
Technical role
Lighting power is one building-model input
The model typically considers lighting power, controls, operating schedules, and interactions with heating and cooling. Reduced fixture heat may lower cooling energy while changing heating demand. A simple fixture-count spreadsheet does not capture every modeled interaction.
Preserve room-by-room quantities, input watts, controls, schedules, and installed substitutions so the model reflects the actual project.
Illustrative lighting power change
Power-density improvement is useful evidence, not the statutory whole-building result.
Hypothetical baseline
Hypothetical installed value
| Item | Value | Note |
|---|---|---|
| Existing lighting | 1.00 W/SF | Hypothetical baseline |
| Installed LED system | 0.55 W/SF | Hypothetical installed value |
Controls
Occupancy, daylight, scheduling, and shutoff strategies need evidence
Control narratives can affect modeled operation when recognized under the applicable method and supported by the installation. Document zoning, sensor coverage, daylight-responsive controls, time schedules, and commissioning results rather than relying on design labels.
Emergency, process, exterior, and tenant lighting may require separate classification. The reviewer should reconcile what is inside the modeled building scope.
Current framework
Do not revive the obsolete partial-lighting shortcut
Older Section 179D guidance included interim lighting rules for certain historical property. Current projects should not be marketed as qualifying through an unchanged watts-per-square-foot shortcut.
For current ordinary-route work, test the building under the current statutory framework and applicable reference standard. For historical work, identify the property period first and then apply the rules in effect at that time.
Lighting file to whole-building model
Connect granular field records to the broader technical analysis.
Fixture schedule
Quantity and watts
Controls
Zones and operation
Field check
Verify installation
Building model
Evaluate interaction
Installed record
Verify what was placed in service
Keep fixture schedules, cut sheets, control diagrams, submittals, invoices, cost ledgers, commissioning records, service evidence, and field counts. Reconcile substitutions and areas omitted from the project.
The resulting per-square-foot amount can still be limited by eligible cost and prior deductions. Bonus treatment also requires a project-specific prevailing-wage and apprenticeship review or applicable exception.
What to reconcile
Small classification differences can change model inputs and eligible cost.
| Review point | Evidence or measure | Why it matters |
|---|---|---|
| Fixtures | Count + wattage | Match schedules, cut sheets, and installed field count. |
| Controls | Type + zone | Document occupancy, daylight, scheduling, and shutoff logic. |
| Areas | Modeled scope | Separate interior, exterior, process, emergency, and omitted areas. |
Prepare the review
Document the installed lighting system room by room
- Existing and proposed fixture schedules
- Cut sheets and input watts
- Room areas and field quantities
- Control types and zones
- Operating schedules
- Emergency and process-lighting classification
- Approved substitutions
- Commissioning and functional testing
- Placed-in-service evidence
- Cost ledger and prior-deduction review
Continue the review
Common questions
Practical answers before the review
Does an LED retrofit automatically qualify?
No. It can improve performance, but the applicable whole-building test and all other requirements still apply.
Can I use the old partial-lighting rule for a current project?
Do not assume so. Historical interim rules should not be carried into current-law projects.
Are utility savings the Section 179D percentage?
No. Utility analyses and the statutory modeled comparison may use different baselines, inputs, and purposes.