LED Retrofit for a Factory: Lux Targets, Fixture Choice and Payback

An LED retrofit in a factory pays back on three numbers: the watts you remove, the hours the lights burn and the unit rate you avoid. Fix the lux target for each task first, from IS 3646 and the National Building Code, then choose fixtures by the light they deliver at the working plane, not by wattage. Payback is installed cost divided by the net annual saving.

What lux should each area target?

IS 3646 (Part 1) sets out the principles of good lighting in buildings, chiefly for working areas in industrial, commercial and public buildings, and the recommended illuminance values sit in its tables and in Part 8 of the National Building Code 2016. They are given by task, and they are maintained values, meaning the light left at the end of the cleaning cycle, not the day-one reading. We do not reproduce the tables here; take each number from the standard or your client's specification, and write its source on the drawing.

Then measure what you have. A lux-meter grid at working height, taken at night or with daylight excluded, shows whether the area is already over-lit. Many older factories turn out to be over-lit in some bays and under-lit in others, and a one-to-one swap copies that unevenness. Set the target by area type:

• General production and assembly bays take their value from the task, with attention to uniformity across work benches.
• Inspection and fine work usually need higher illuminance and better colour rendering, which affects the fixture choice, not only the count.
• Warehouse aisles and racking need light on the vertical face of the racks, so the optics and mounting height matter more than the average lux.
• Yards, gates and walkways need security and safety levels, with controls to avoid burning all night.
• Emergency and escape lighting is a separate system with its own requirements and is not covered by the retrofit calculation.

Which fixture replaces which?

How do you choose a fixture, apart from price?

Compare bids on delivered light, not nameplate watts. The datasheet should state luminous flux for the whole luminaire, the photometric file (IES or LDT) for your layout software, the driver's power factor and harmonic distortion, the ingress-protection rating, the operating temperature range and the warranty term and what it excludes. Ask whether the product has the BIS registration your buyer's policy requires, and for the lumen-maintenance data behind the stated life.

Two factory-specific points are often missed. First, power quality: low-grade drivers add harmonics and can lower the power factor, which matters if your DISCOM bills on kVAh, because the billed units rise as power factor falls. Second, flicker near rotating machinery, where a poor driver can create a stroboscopic effect. A trial area for two weeks costs little compared with a rejected order.

For a first fitting count, the lighting lux calculator applies the lumen method: fittings needed = (target lux x area) / (lumens per fitting x utilisation factor x maintenance factor). It rounds up to whole fittings and shows the installed load. Treat the answer as a screening estimate until a photometric layout checks uniformity and glare.

How do you work out the payback?

The method has four lines:

1. Annual energy saved (kWh) = (old system watts minus new watts) x number of fittings x burning hours per year / 1,000. Use system watts, meaning the lamp plus the ballast, choke or driver; a tube labelled 36 W on a magnetic choke draws more than 36 W.
2. Annual saving (₹) = energy saved x the energy charge you avoid, with any per-unit duty, less any increase in maintenance cost, plus any fall in maintenance.
3. Installed cost = fittings, installation, access equipment and disposal of the old fittings.
4. Simple payback = installed cost / net annual saving.

Burning hours dominate. The table shows the energy and money saved for every 100 W removed, using ₹9 per unit, the avoided-energy assumption on our solar size pages; replace it with your own tariff.

So a fitting that sheds 150 W in a continuous-process bay saves about 1,200 kWh a year, and the same fitting in a single-shift bay saves about 375 kWh. Put your installed cost per fitting against that saving, area by area, and the retrofit order follows: start with the longest-burning areas. The LED retrofit savings calculator handles groups, maintenance change and kVAh conversion, and shows the payback sensitivity if hours or tariff move.

What moves the installed cost?

We do not publish a price for lighting retrofits, because the cost depends on drivers of the following kind, which you should have itemised in the BOQ:

• Mounting height and access: scissor lift or tower for tall bays, and shutdown windows.
• Whether the swap is one-to-one or changes the layout and circuits.
• Wiring changes: removing chokes, replacing damaged holders, adding circuits for control.
• Controls: photocells, occupancy sensors, dimming and zoning.
• Emergency and exit fittings if the retrofit touches them.
• Disposal of old fittings and discharge lamps, handled as your waste rules require.
• Warranty length and what is excluded.

Do the efficiency work before sizing solar. Lower lighting load shrinks the base load a solar plant must match, so use the corrected load in the solar calculator.

How Secured Engineers helps

We design and execute factory electrical infrastructure in-house, including lighting circuits, panels and power-factor correction, so a lighting retrofit is checked against the panel and the sanctioned load it sits on. We also file Fire NOC, CEIG and DISCOM net-metering approvals ourselves, and MEPF and solar can sit under one contract. We work from offices in Noida (Delhi NCR) and Ludhiana, with 535+ projects across 18+ states and 89.77% repeat clients.

If you have a lighting BOQ or a supplier quote, send it for a budget review: we check the lux basis, system watts, hours and the exclusions before you commit. Use WhatsApp or call +91 70099 85817, or ask for a free proposal from the electrical infrastructure team.