--- title: "Electronic ballast replacement savings with ToD tariff" subject: "Industrial Power Distribution and Illumination" chapter: 10 key: "semester_6/industrial_power_distribution_and_illumination:ballast-replacement-tariff" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/ballast-replacement-tariff site: BCT Engineering --- # Electronic ballast replacement savings with ToD tariff Industrial Power Distribution and Illumination · Chapter 10: Electrical Energy Audit in Industry - Asked in: 2070 Magh, 8 marks - 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/ballast-replacement-tariff - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-10 ## Question In an industry, there are 100 numbers of 1×36 watt fluorescent tubes and 200 numbers of 2x 36 watt fluorescent tubes with copper ballast. The p.f. with copper ballast is very poor and is below 0.5. Also, losses in that ballast are very high. Replacing the electronic ballast can avoid these two problems. By laboratory test conducted on 36W fluorescent tube followings are finding: (a) with cu. Ballast power consumed 44W and p.f. is 0.5 b) with electronic ballast power consumed 37 watt and p.f 0.9. Calculate the energy cost saving and payback period with electronic ballast; if operating hours are 8 hours/day and electronic ballast cost is NRs 300 per piece, energy tariff is: 6 AM - 6 PM, Rs.5.75 paisa 6 PM - 11 PM, Rs.6.55 paisa 11 PM - 6 AM, Rs.4.00 paisa And kVA charge per month Rs. 210.00 paisa ## Short answer Replacing 500 tubes with electronic ballasts saves 3.5 kW and 23.44 kVA. Annual energy saving Rs 58,765 (day tariff), demand saving Rs 59,080; total Rs 1.18 lakh. Payback ≈ 1.27 years. ## Answer **Assumptions:** one ballast per tube (electronic ballast cost Rs 300 per tube), lights run 8 h/day for 365 days, and all 8 h fall in the 6 AM – 6 PM band (day shift, Rs 5.75/kWh). The lights are on during the monthly peak, so the kVA saving reduces the demand charge. **Number of tubes:** ```text 100 × (1×36 W) = 100 tubes 200 × (2×36 W) = 400 tubes Total N = 500 tubes ``` **Power and kVA per tube:** | Ballast | kW per tube | pf | kVA per tube | |---|---|---|---| | Copper | 0.044 | 0.5 | 0.044/0.5 = 0.0880 | | Electronic | 0.037 | 0.9 | 0.037/0.9 = 0.0411 | **Total load:** ```text Copper: P = 500 × 44 = 22.0 kW, S = 44.00 kVA Electronic: P = 500 × 37 = 18.5 kW, S = 20.56 kVA Saving: ΔP = 3.5 kW, ΔS = 23.44 kVA ``` **Annual energy cost saving:** ```text ΔE = 3.5 kW × 8 h × 365 = 10 220 kWh/year Cost saving = 10 220 × 5.75 = Rs 58 765 /year ``` **Annual demand charge saving:** ```text = 23.44 kVA × 210 × 12 = Rs 59 080 /year ``` **Total saving and payback:** ```text Total saving = 58 765 + 59 080 = Rs 117 845 /year Investment = 500 × 300 = Rs 150 000 Payback = 150 000 / 117 845 = 1.27 years ≈ 15.3 months ``` If the 8 h were instead in the evening peak (Rs 6.55/kWh), the energy saving would be 10 220 × 6.55 = Rs 66 941 and payback ≈ 1.19 year; the conclusion is the same. Other benefits: lower line current (less cable and transformer loss), no flicker or hum, and longer tube life with electronic ballasts. **Answer: Saving ≈ Rs 1.18 lakh per year (Rs 58 765 energy + Rs 59 080 demand); payback ≈ 1.27 years.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How many tubes (and so ballasts) are counted for 100 x (1x36 W) and 200 x (2x36 W) fittings? A. 300 B. 700 C. 400 D. 500 Answer: D. The 100 single-tube fittings give 100 tubes and the 200 twin-tube fittings give 400, total 500 tubes with one ballast each. (Key point: Number of tubes) 2. How is the kVA saving per tube obtained? A. kVA = kW / pf for each ballast type, then difference B. kVA = kW x pf C. Difference of kW only D. kVA = kW / (1 - pf) Answer: A. Copper: 0.044/0.5 = 0.088 kVA; electronic: 0.037/0.9 = 0.0411 kVA. For 500 tubes the saving is 23.44 kVA. (Key point: kVA from pf) 3. Which two savings make up the total annual saving? A. Energy (kWh) saving only B. Energy cost saving from 3.5 kW less power and kVA demand charge saving C. Ballast resale and lamp cost D. Reduced operating hours and tariff change Answer: B. Energy saving: 3.5 kW x 8 x 365 x 5.75 = Rs 58 765; demand saving: 23.44 x 210 x 12 = Rs 59 080. (Key point: Two savings) 4. With investment 500 x Rs 300 = Rs 150 000 and total saving Rs 117 845/year, what is the payback? A. About 0.5 year B. About 1.27 years C. About 2.5 years D. About 12 years Answer: B. Payback = 150 000 / 117 845 = 1.27 years. 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