Lighting retrofit and payback in a workshop
A 60 m × 30 m workshop with an 8 m mounting height requires a maintained illuminance of 500 lux on the working plane. The existing installation has 120 high-bay metal-halide luminaires, each taking 440 W including the magnetic ballast, and operates 4,000 h per year. The owners propose replacing them with 150 W LED high-bay luminaires having an efficacy of 150 lm/W. The utilization factor is 0.65 and the maintenance factor is 0.70. The electricity tariff is ₹8.50 per kWh. Each LED luminaire costs ₹12,000 and installation adds ₹500. The existing metal-halide lamps have a rated life of 12,000 h and cost ₹1,800 each; LED life is 50,000 h. The supply has a high neutral current and the owner is also considering a harmonic filter. The workshop operates five days a week.
1. Calculate the number of LED luminaires required using the lumen method and check the achieved illuminance. Is the retrofit likely to be over- or under-lit compared with the existing installation?
Read the worked answer
Room area = 60 × 30 = 1800 m².
Required lumens = E × A / (CU × MF) = 500 × 1800 / (0.65 × 0.70) = 900,000 / 0.455 ≈ 1,978,022 lm.
Each LED gives 150 W × 150 lm/W = 22,500 lm.
N = 1,978,022 / 22,500 ≈ 87.9, so use 88 luminaires.
Achieved maintained illuminance = 88 × 22,500 × 0.65 × 0.70 / 1800 ≈ 500.5 lux. That meets the target with a small margin.
The existing 120 metal-halide fittings are likely over-lighting the space. Even at a conservative 32,000 lm per 400 W lamp, the existing maintained illuminance would be roughly 120 × 32,000 × 0.455 / 1800 ≈ 970 lux, about twice the requirement. Actual lamp output and maintenance condition will vary, so a light-meter survey is needed, but the retrofit is not likely to be under-lit; it is a rational reduction to the design level.
2. Calculate annual energy and cost savings and the simple payback. Include lamp replacement savings if relevant.
Read the worked answer
Existing energy = 120 × 0.440 kW × 4000 h = 211,200 kWh/year.
New energy = 88 × 0.150 kW × 4000 h = 52,800 kWh/year.
Energy saving = 158,400 kWh/year.
Cost saving = 158,400 × ₹8.50 = ₹1,346,400/year.
Capital cost = 88 × (₹12,000 + ₹500) = ₹1,100,000.
Simple payback = 1,100,000 / 1,346,400 ≈ 0.82 years, about 9.8 months.
Lamp replacement adds to the case. Existing lamps last 12,000 h, or 3 years at 4,000 h/year, so about 40 lamps are replaced per year: 40 × ₹1,800 = ₹72,000/year, before labour. Including that, the annual saving is about ₹1,418,400 and payback falls to about 0.78 years. This is a strong financial case, but the calculation assumes the LED luminaires achieve their rated efficacy in the actual thermal environment and that the existing installation is otherwise functional.
3. Identify two non-energy benefits and one technical risk that should be checked before approving the retrofit.
Read the worked answer
Non-energy benefits include lower maintenance at 8 m mounting height - no frequent lamp changes and less access-equipment risk - and instant restrike with better light uniformity and dimming or daylight-harvesting capability. There is also less heat rejected into the workshop, which reduces air-conditioning load where applicable.
The technical risk is power quality. LED drivers are nonlinear and can inject harmonics, particularly third harmonic, into a supply that already has high neutral current. The retrofit could make the neutral loading worse unless low-THD drivers are specified and the neutral is checked. Verify the driver THD, inrush current, power factor and immunity to the existing voltage distortion. Also check glare at 8 m, because a narrower beam LED replacement can create uncomfortable glare even at the correct lux level. Require a photometric design and a trial measurement before full replacement, and do not assume that adding a harmonic filter later will be free.