--- title: "Energy and cost saving from lighting retrofit" subject: "Industrial Power Distribution and Illumination" chapter: 10 key: "semester_6/industrial_power_distribution_and_illumination:lighting-retrofit-100kw" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lighting-retrofit-100kw site: BCT Engineering --- # Energy and cost saving from lighting retrofit Industrial Power Distribution and Illumination · Chapter 10: Electrical Energy Audit in Industry - Asked in: 2068 Magh (old course), 8 marks - Video: [Lighting Retrofits: The 4-Step Math to Save Money! #energyengineering](https://www.youtube.com/watch?v=OnzBPWtg2Ls) — Prof.K. - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lighting-retrofit-100kw - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-10 ## Question An industry has a lighting load of 100kW as: a) 60kW of 40W fluorescent lamp each with copper ballast b) 40kW of incandescent lamp with 200W each The lights are at present being used for 3500 hrs/year. Following an energy survey, the plant has decided to change all copper ballast to electronic ballast and all incandescent lamp to fluorescent lamp with electronic ballast. The energy awareness program is expected to result in 10% reduction in lighting hours. Calculate the energy and cost saving considering the following factors: a) The price of electricity is Rs. 7.50 / kWh b) The power consumption of copper ballast and electronic ballast are 8W and 1W respectively c) The cost of electronic ballast is Rs. 500.00/piece d) The cost of FTL fixture is Rs. 900.00 per set with electronic ballast The lamp life of fluorescent lamp and incandescent lamp are 8000 hrs and 1000 hrs respectively. ## Short answer Replacing copper ballasts with electronic ballasts and incandescent lamps with fluorescent lamps reduces load from 112 kW to 69.7 kW. Annual energy saving = 172,445 kWh; cost saving = Rs 1,293,338. Investment Rs 930,000, payback 0.72 years. ## Answer **Assumptions:** the 60 kW and 40 kW are lamp wattages (ballast loss extra); each 200 W incandescent lamp is replaced by one 40 W fluorescent lamp with electronic ballast (similar lumen output); lamp prices are not given, so lamp replacement cost is shown only as number of lamps. **Number of lamps:** ```text Fluorescent = 60 000 / 40 = 1500 lamps Incandescent = 40 000 / 200 = 200 lamps ``` **Present load (3500 h/year):** ```text FTL + copper ballast = 1500 × (40 + 8) = 72.0 kW Incandescent = 200 × 200 = 40.0 kW Total = 112.0 kW Energy = 112 × 3500 = 392 000 kWh/year ``` **Proposed load (hours reduced 10 %: 3150 h/year):** ```text All FTL + electronic ballast = (1500 + 200) × (40 + 1) = 1700 × 41 = 69.7 kW Energy = 69.7 × 3150 = 219 555 kWh/year ``` **Saving:** ```text Energy saving = 392 000 − 219 555 = 172 445 kWh/year Cost saving = 172 445 × 7.50 = Rs 1 293 338 /year ``` **Investment and payback:** ```text Electronic ballasts = 1500 × 500 = Rs 750 000 New FTL fixtures = 200 × 900 = Rs 180 000 Total investment = Rs 930 000 Payback = 930 000 / 1 293 338 = 0.72 year ≈ 8.6 months ``` | Item | Before | After | |---|---|---| | Connected load | 112.0 kW | 69.7 kW | | Hours/year | 3500 | 3150 | | Energy/year | 392 000 kWh | 219 555 kWh | | Cost/year | Rs 2 940 000 | Rs 1 646 663 | **Lamp replacement benefit:** ```text Incandescent: 200 × 3500/1000 = 700 lamps/year FTL (new 200): 200 × 3150/8000 = 78.75 ≈ 79 lamps/year ``` About 620 fewer lamp replacements per year, plus less maintenance labour and less heat in the building. **Answer: Energy saving ≈ 172 445 kWh/year, cost saving ≈ Rs 12.93 lakh/year, investment Rs 9.3 lakh, payback ≈ 0.72 year.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the present fluorescent load found including ballast loss? A. 1500 x 40 W only = 60 kW B. 1500 x (40 + 8) W = 72 kW C. 1500 x (40 + 1) W = 61.5 kW D. 1500 x 8 W = 12 kW Answer: B. The 60 kW is lamp wattage and ballast loss is extra, so each fitting draws 48 W, giving 72 kW; with 40 kW incandescent the total is 112 kW. (Key point: Present load with ballast loss) 2. What are the proposed load and hours after the retrofit and awareness program? A. 69.7 kW for 3150 h/year B. 112 kW for 3150 h/year C. 69.7 kW for 3500 h/year D. 60 kW for 3850 h/year Answer: A. 1700 lamps x 41 W = 69.7 kW, and a 10% cut in hours gives 3150 h/year. (Key point: Proposed load and hours) 3. What makes up the investment of Rs 9.3 lakh? A. 1500 ballasts at Rs 900 B. 200 incandescent lamps at Rs 500 C. 1700 ballasts at Rs 500 and fixtures D. 1500 electronic ballasts at Rs 500 plus 200 new FTL fixtures at Rs 900 Answer: D. Rs 750 000 for the 1500 ballasts plus Rs 180 000 for 200 fixtures replacing incandescent lamps. (Key point: Investment) 4. Why does the retrofit reduce lamp replacements for the 200 replaced lamps? A. Fluorescent lamp life is 8000 h against 1000 h for incandescent B. Incandescent lamps last longer C. Hours of use increase D. Ballast loss decreases Answer: A. Incandescent need 200 x 3500/1000 = 700 lamps/year, while FTL need only about 79, a saving of roughly 620 replacements. (Key point: Lamp life benefit) ## Related in Industrial Power Distribution and Illumination - [Electronic ballast replacement savings with ToD tariff](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aballast-replacement-tariff) — Industrial Power Distribution and Illumination, chapter 10 - [Process room 30 × 15 × 5 m fluorescent lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aprocess-room-30x15-fluorescent) — Industrial Power Distribution and Illumination, chapter 7 - [Industry room 30 × 20 m LED lighting and DB](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aindustry-room-30x20-led) — Industrial Power Distribution and Illumination, chapter 7, asked 2× - [Types of lighting scheme in industry](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aindustrial-lighting-schemes) — Industrial Power Distribution and Illumination, chapter 7, asked 4× - [Department store 30 × 15 m twin 40 W fluorescent](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Adepartment-store-twin-40w-fluorescent) — Industrial Power Distribution and Illumination, chapter 7, asked 3× - [Process room 40 × 20 m twin 40 W fluorescent](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aprocess-room-40x20-fluorescent) — Industrial Power Distribution and Illumination, chapter 7, asked 2× ## Related in other subjects - [Energy saving calculation: lighting and motor](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Alighting-motor-energy-saving-case) — Energy Resources, chapter 9 - [Annual cost and saving with pf correction (400 kVA)](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aindustry-50000-units-pf-saving) — Utilization of Electrical Energy, chapter 6 - [Tariff types used in industry billing by NEA](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Atariff-types-nea-industry) — Utilization of Electrical Energy, chapter 6 - [Copper saving in an autotransformer](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aautotransformer-copper-saving) — Electrical Machine, chapter 2, asked 2×