- 2068 Magh (old course) · 8 marks
Energy and cost saving from lighting retrofit
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.
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.
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:
Fluorescent = 60 000 / 40 = 1500 lamps
Incandescent = 40 000 / 200 = 200 lamps
Present load (3500 h/year):
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):
All FTL + electronic ballast = (1500 + 200) × (40 + 1)
= 1700 × 41 = 69.7 kW
Energy = 69.7 × 3150 = 219 555 kWh/year
Saving:
Energy saving = 392 000 − 219 555 = 172 445 kWh/year
Cost saving = 172 445 × 7.50 = Rs 1 293 338 /year
Investment and payback:
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:
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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