LED relamping: trade-offs between energy performance and usage constraints
3 structuring trade-offs in office LED relamping
On an office floor properly equipped with electronic T5 fixtures installed after 2010, full LED relamping is rarely the right lever: DALI control alone delivers substantial savings for a fraction of the CAPEX. In renovation, installed power must stay below 2.8 W/m² per 100 lux of average illuminance to be maintained, a ceiling that low-cost LED degrades within 3 years for lack of photometric stability. A successful LED relamping project is not just about replacing fixtures one for one. The approach involves 3 interlocking technical dimensions: real photometry of the spaces, existing electrical architecture, and occupancy profiles differentiated by zone. The energy savings observed in office LED relamping generally range between 50 and 75% depending on sector benchmarks, with a return on investment of 3 to 5 years depending on the chosen control density. Kytom has been working on these projects since 2006 with a systematic preliminary audit covering 5 points (consumption, lux measurement, hourly profiles, electrical condition, control scenarios), the prerequisite for mastering the 10-year TCO.
Every LED relamping project involves 3 technical tensions that the audit phase must address before specification:
- Energy performance vs visual comfort. Maximum luminous efficacy (up to 150 lm/W on certified fixtures) is not enough. Colour temperature determines perceived comfort: 3000K for relaxation and reception areas, 4000K for focused offices, with a colour rendering index (CRI) above 80 required at office workstations.
- Initial investment vs 10-year TCO. Premium fixtures show a noticeably higher purchase cost, but their extended lifespan and superior lumen maintenance over 50,000 hours significantly reduce maintenance costs over 10 years compared with entry-level references.
- Standardisation vs flexibility by zone. A single solution simplifies technical management but ignores differentiated needs. The occupancy ratios observed in office spaces place the usual density between 8 and 12 m² per workstation in open space and 12 to 18 m² in closed offices, which requires distinct sizing by type.
The preliminary audit arbitrates these tensions according to measured usage profiles, never on declared assumptions.
Our reading differs from the professional orthodoxy on this precise point: full relamping is not the default answer. Below a certain surface threshold with a recent installation of high-efficiency electronic T5 fixtures, the return on investment of full relamping can exceed several years, making control alone (detection, dimming) more relevant than replacing the fixtures. The energy target is then addressed through control alone (detection, dimming) without changing fixtures. Likewise, on a site whose sale or relocation is scheduled within less than 4 years, full relamping is counterproductive and a targeted retrofit on intensive-use zones is sufficient.
4 recurring mistakes that compromise the return on investment
Analysis of our office relamping projects reveals 4 recurring methodological failures that erode the expected gains:
- Photometric audit skipped. Replacing lux for lux without surveying the light distribution creates under-lit or glaring zones (UGR above 19, beyond the admissible limit for computer workstations).
- Electrical constraints underestimated. Old circuits sometimes require DALI adapters, partial renovations of distribution boards or zonal rewiring, which can significantly increase the initial budget.
- Selection on purchase price alone. Entry-level LEDs see their luminous flux degrade markedly faster than certified references, undermining the real TCO.
- Undifferentiated management by zone. Applying the same solution to circulation areas (200 lux required) and workstations (500 lux) creates usage mismatches and energy oversizing.
| Space type | Required level | Recommended colour temperature |
|---|---|---|
| Circulation areas, passageways | 100 to 200 lux | 3000K |
| Open space, offices | 300 to 500 lux | 4000K |
| Meeting rooms | 300 to 500 lux | 3500K to 4000K |
| Technical rooms | 500 to 750 lux | 4000K |
Segmenting the audit by type determines the relevance of the sizing.
Kytom methodology: 4 steps to secure energy savings
The Kytom approach structures LED relamping into 4 sequenced phases, validated on the office projects in our portfolio:
- Energy and photometric audit (1 to 2 weeks). Measurement of current consumption at the distribution meter, lux survey by zone, mapping of hourly usage profiles. This phase identifies the real savings potential and sets aside theoretical assumptions.
- Differentiated technical sizing. Calculation of needs by type according to the indoor lighting standard for workplaces, selection of optics (asymmetric in circulation areas, micro-prismatic in offices), definition of the control strategy (DALI, presence detection, daylight dimming).
- Phased planning. Sequencing of the works to maintain activity (interventions by floor, outside working hours if necessary), coordination with the other technical packages (HVAC, ceilings, low-voltage systems), staggered supply.
- Commissioning and fine-tuning. Verification of measured levels, calibration of control scenarios, training of preventive maintenance teams.
For the architect and lighting designer: the focus shifts. The trade-off is no longer about the choice of an isolated fixture but about the photometric consistency of the floor over time. The applicable standard sets UGR values of RUGL=16 for high requirements, RUGL=19 to 25 for medium requirements and RUGL=28 for low requirements, but it is the zonal control strategy and the management of the maintenance factor that determine the durability of the architectural design. Beyond 5 usage reconfigurations planned over 10 years, addressable DALI control becomes a priority over the choice of the fixture itself.
Limits of the method. This 4-step sequence loses its value in three cases: single-function surfaces below 200 m² (direct sizing by chart is enough), sites on precarious leases of less than 24 months (the investment cannot be amortised), buildings undergoing simultaneous major renovation of the building envelope (relamping must then be integrated into the overall power-supply package, not handled independently).
Multi-site intervention framework: 11 agencies and controlled timeframes
LED relamping frequently fits into multi-site programmes subject to the tertiary decree, which requires energy consumption reduction trajectories of 40% by 2030 compared with a reference year. Kytom supports these rollouts from its 11 agencies in France and Spain with a single technical reference framework (fixtures and control specifications in 6 sections), a single programme lead and consolidated zone-by-zone reporting.
Frequently asked questions
When is full LED relamping relevant compared with control alone?
Full LED relamping becomes relevant beyond 300 m² of homogeneous floor space fitted with pre-2010 luminaires or ferromagnetic T5. On a post-2010 electronic T5 installation, DALI control alone (detection, dimming) delivers most of the savings for a fraction of the CAPEX. For a site scheduled for disposal within 4 years, a targeted retrofit of high-usage zones is sufficient.
