On an industrial site, the question "how much does your lighting consume?" almost always goes without a figure in reply. It is not negligence. Lighting is rarely sub-metered, it never fails all at once, and it disappears into the "general electricity" item on the bill. The result: no up-to-date inventory of the installed base, and no record of the hours the lights are really on.
The problem becomes concrete as soon as a decision has to be made. Without a measured basis, a renovation project is built on catalogue ratios and promises of savings in percentages. Both are contradicted by the first field survey. The method that holds is the opposite: measure first, cost afterwards.
Why estimating by eye is almost always wrong
Three errors come up on almost every site we visit.
- Nominal power is not absorbed power. A 58 W fluorescent tube on a magnetic ballast draws noticeably more than 58 W once the control gear is counted. Over several hundred light points, the difference stops being anecdotal.
- The installed base is no longer uniform. Fifteen or twenty years of piecemeal replacements produce mixtures: original metal halide, a few emergency LED replacements, tubes of different wattages in the same bay, optics dirty to very different degrees.
- Operating hours are underestimated. Operations reply "we switch on at 6 am, we switch off at 10 pm". The survey often shows zones lit continuously, weekends included, because the switch controls three bays at once and no one wants to plunge a colleague into darkness.
The consumption of a luminaire is a power multiplied by a duration. When both terms are approximate, the result is worthless, and so is the profitability study built on it.
Step 1: the inventory of the installed base, luminaire by luminaire
This is the thankless part, and it determines everything else. The inventory is done on site, not over the phone. For each zone, we record:
- the exact number of light points, distinguishing those out of service or removed;
- the technology and power of the source, control gear included;
- the mounting height and the layout grid;
- the type of optic and its condition: oxidised reflector, yellowed diffuser, opaque cover;
- the constraints of the zone: temperature, dust, washing, corrosion, explosive atmosphere, storage height;
- the type of control: switch, contactor, timer, detection, BMS;
- the presence and condition of emergency lighting, which follows its own logic.
The mounting height immediately points to the family of solutions. Between 6 and 12 metres, the reasoning is in terms of LED high bays whose optic is chosen according to the width of the aisles. Above that, in assembly halls or high-bay stores, you need luminaires designed for great heights, with tight angles and suitable thermal performance. Below 5 metres, in low workshops, circulation areas and preparation zones, LED linear battens remain the most effective answer, especially when an existing layout has to be reworked without drilling everywhere.
A plan, not just a spreadsheet
The inventory must be transferred onto a plan. This is what later makes it possible to reason in grids rather than in units: you do not replace one luminaire with another, you redistribute flux within a volume. The difference shows from the first calculation.
Step 2: measuring with a lux meter, under operating conditions
Measurement is done under real working conditions, machines in place, racks full, pallets where they usually are. A hall measured empty gives results that operations will never see.
In practice: a grid of points on the working plane, generally 0.85 m for a workstation and the floor for a circulation area, measurements at night or with daylight neutralised, and a record of the average illuminance as well as the minimum. The value that counts is not the average alone but the uniformity: a zone averaging 400 lux with dips at 90 lux is a badly lit zone, whatever the average shown.
The reference values come from NF EN 12464-1 for indoor spaces, which sets requirements by type of task: around 200 lux for a circulation area or storage, 300 lux for an ordinary workstation, 500 lux and more for fine work, visual inspection or a rework station. Added to this are glare, colour rendering and, for quality control stations, a spectrum requirement. NF X35-103 deals with visual ergonomics, the Labour Code imposes the obligation of result, and emergency lighting falls under NF EN 1838, to be checked separately, including when it is built into the general luminaires.
What the measurement almost always reveals
- Oversized zones: circulation aisles, docks, plant rooms lit like workstations.
- Workstations lit below the standard values, often the most demanding ones, compensated by desk lamps bought by the teams.
- Uniformity degraded by successive replacements and by dirt, very uneven from one luminaire to another.
- Discomfort glare in zones where people work looking up: overhead cranes, cabins, maintenance at height.
Step 3: recording the real operating hours
This is the item where estimates go most wrong, and it is the one that weighs most heavily on the bill. Three sources are cross-checked: a current logger fitted on the lighting circuits of the switchboard for one to two weeks including a weekend, the BMS readings where they exist, and interviews with the shift teams, who know what stays on at night.
Operating pattern | Order of magnitude of hours lit per year | Point to check in the survey |
|---|---|---|
Single day shift, 5 days | About 2,000 h | Switch-offs during breaks actually practised |
Two shifts | About 4,000 h | Zones left on between shifts |
Three shifts | 6,000 to 7,000 h | Weekends and technical shutdowns |
Continuous process or zone without local control | Up to 8,760 h | Emergency lighting and standby zones |
These orders of magnitude only serve to frame the interview. The value used in the calculation must come from the survey, zone by zone. A difference of 1,500 hours a year on a hall of two hundred luminaires is enough to change the payback time by more than a year.
Step 4: ranking the sources of savings
Three levers act, and rarely in the same zones:
- Installed power, when the technology is old and the grid is oversized.
- Duration, when little-used zones stay lit permanently: presence detection in storage aisles, zoning of controls, dimming according to daylight under sheds or rooflights.
- The number of points, when the photometric calculation shows that a revised layout meets the same requirement with fewer luminaires.
The ranking is done in euros per year, not in percentage savings. A zone that consumes little does not deserve the same effort as a store lit continuously, even if the percentage saving there looks spectacular.
A costed example: an airport arrivals hall
The case illustrates what the method produces. The existing installation had 398 luminaires. After surveying the levels, measuring the hours and calculating the photometry, the chosen layout has 315, that is 83 points removed.
Indicator | Before | After |
|---|---|---|
Number of luminaires | 398 | 315 |
Annual consumption | €12,197 | €3,340 |
Annual saving | €9,027 | |
Project budget | €27,405 | |
Payback time | 37 months | |
Net gain over 5 years | €16,880 |
Luminaires are not removed to save money. They are removed because the calculation shows that the illuminance requirement is met with fewer points, better placed. The saving is the consequence, not the objective of the sizing.
Other operations of this kind appear in our projects, on industrial, logistics and tertiary sites.
Financing: what changed in 2026
A regulatory point to know before building a financing plan. The order of 23 February 2026, in force since 25 February 2026, abolished the CEE sheets BAR-EQ-110, BAT-EQ-127 and IND-BA-116. There is therefore no longer a standardised bonus for indoor lighting renovation. Any claim to the contrary, in a quote or a brochure, dates from before this deadline.
What can still be used:
- Outdoor lighting renovation remains eligible through sheet RES-EC-104, revised by the order of 24 November 2025. Car parks, private roads and building surroundings fall within this scope, with outdoor and road lighting luminaires sized according to the NF EN 13201 series.
- Long-term leasing over 3 to 5 years, the most common arrangement today for indoor lighting: the monthly saving covers the instalment, and the operation is carried out without tying up investment.
- The related items still eligible, often handled in the same operation: BMS, destratification, variable speed drives.
Why the audit and the study are free
At Luminance Consulting, the lighting audit and the photometric study are free, site visit and 3D modelling included. The reason is simple and we say it frankly: we are paid on the solution we supply and have installed, not on the engineering. Our design office is one skill among others, in the same way as field expertise, the choice of luminaires, the financing plan and follow-up through to commissioning and the check of levels.
The deliverable is the same whatever the next step: inventory of the installed base, lux meter readings, measured operating hours, compliance calculations to NF EN 12464-1, planned layout and payback time zone by zone. You know what you consume, and why. It is this document that makes it possible to decide, including deciding that a zone does not justify any work.
Do you want to know what your lighting really consumes before committing to any expenditure? Contact us to schedule the visit. The audit and the photometric study are free, and we work throughout France, with a site visit for the survey.
