---
title: "CFL versus filament bulb economic comparison"
subject: "Engineering Economics"
chapter: 4
key: "semester_6/engineering_economics:cfl-vs-filament-bulb"
source: https://www.bctengineer.com/q/semester_6/engineering_economics/cfl-vs-filament-bulb
site: BCT Engineering
---

# CFL versus filament bulb economic comparison

Engineering Economics · Chapter 4: Comparative Analysis of Alternatives

- Asked in: 2069 Poush
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/engineering_economics/cfl-vs-filament-bulb
- Whole chapter: https://www.bctengineer.com/md/semester_6/engineering_economics/chapter-4

## Question

If the cost of a 25 watt CFL bulb is Rs. 260 whereas the cost of a 100 watt filament bulb is Rs. 35 but these bulbs have equal lighting power, which bulb do you prefer in your use and why? When electricity cost is Rs. 11 per unit (kw-hr).

## Short answer

Prefer the 25 W CFL bulb. Its higher purchase price is quickly offset by lower electricity usage: annual energy cost is Rs. 412 for CFL vs Rs. 1,650 for filament, yielding a total annual cost of Rs. 495 compared to Rs. 1,702.

## Answer

The bulb with the lower total yearly cost for the same light is the better buy. Electricity use, not purchase price, dominates here.

#### Assumptions (not given in the question)
- Both bulbs give the same light and are used 1,500 hours per year (about 4 hours a day).
- Rated life: CFL = 6,000 hours (4 years), filament bulb = 1,000 hours (1.5 bulbs per year).
- Interest rate (MARR) = 10%. Electricity = Rs. 11 per kWh.

#### Energy cost per year
- CFL: 25 W = 0.025 kW → 0.025 × 1,500 × 11 = Rs. 412
- Filament: 100 W = 0.1 kW → 0.1 × 1,500 × 11 = Rs. 1,650

#### Equivalent annual cost
CFL: purchase Rs. 260 spread over 4 years, (A/P, 10%, 4) = 0.3155
AEC(CFL) = 260(0.3155) + 412 = 82.0 + 412 = **Rs. 495 per year**

Filament: 1.5 bulbs × Rs. 35 = Rs. 52.5 per year (replacement almost yearly, interest on it is small)
AEC(filament) = 52.5 + 1,650 = **Rs. 1,702 per year**

| Item | CFL 25 W | Filament 100 W |
|---|---|---|
| Purchase cost | 260 | 35 |
| Bulbs needed per year | 0.25 | 1.5 |
| Energy cost/year | 412 | 1,650 |
| Total annual cost | 495 | 1,702 |

Saving by using CFL = 1,208 per year, about 71%.

**Answer: Prefer the 25 W CFL.** Its higher price of Rs. 260 is recovered after about 273 hours of use (roughly 2 months at 4 hours a day), because it saves Rs. 825 of electricity per 1,000 hours. It also needs fewer replacements and gives off less heat.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What criterion decides between bulbs?
   A. Lowest total annual cost for equal light
   B. Lowest purchase price
   C. Longest life only
   D. Lowest wattage rating only
   Answer: A. Same light, so compare total yearly cost including energy. (Key point: Selection criterion)

2. Energy cost per year for the 25 W CFL at 1,500 h and Rs 11/kWh?
   A. Rs 1,650
   B. Rs 82
   C. Rs 412
   D. Rs 260
   Answer: C. 0.025 x 1,500 x 11 = 412. (Key point: Energy cost)

3. Which is preferred and why?
   A. CFL, since energy savings far outweigh higher price
   B. Both equal
   C. Filament, since cheaper
   D. Filament, shorter life
   Answer: A. CFL costs about Rs 495/yr vs Rs 1,702/yr. (Key point: Preferred bulb)

## Related in Engineering Economics

- [Forecast CFL demand for next five years](https://www.bctengineer.com/question.md?key=semester_6%2Fengineering_economics%3Aforecast-cfl-demand-five-years) — Engineering Economics, chapter 1
- [Sensitivity on initial cost and life; interpret](https://www.bctengineer.com/question.md?key=semester_6%2Fengineering_economics%3Asensitivity-30-initial-cost-life-25) — Engineering Economics, chapter 6
- [Sunk cost, ESL, replacement strategy from AEC table](https://www.bctengineer.com/question.md?key=semester_6%2Fengineering_economics%3Asunk-cost-esl-replacement-aec-table) — Engineering Economics, chapter 5, asked 2×
- [Modified B/C ratio for 100,000 project](https://www.bctengineer.com/question.md?key=semester_6%2Fengineering_economics%3Amodified-bcr-100000-25000) — Engineering Economics, chapter 3
- [Cost variance analysis from standard and actual](https://www.bctengineer.com/question.md?key=semester_6%2Fengineering_economics%3Acost-variance-analysis-9000-units) — Engineering Economics, chapter 1
- [Fixed overhead cost variance from standard card](https://www.bctengineer.com/question.md?key=semester_6%2Fengineering_economics%3Afixed-overhead-variance-standard-card) — Engineering Economics, chapter 1

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