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CFL versus filament bulb economic comparison
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.
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).
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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