Earth’s Changing Environment Lecture 19 Lighting Efficiency Problem

We replace 25 incandescent light bulbs rated at 100 Watt each by 25 compact fluorescent bulbs rated at 20 Watts each. The lifetime of the fluorescent bulbs is 10,000 hours of operation.

1. How much energy (in kWh) is consumed by the 25 incandescent 100 Watt bulbs during 10,000 hours of operation (their lifetime)? Energy Consumed = = 25 bulbs 100 W/bulb 10,000 h = 25,000,000 Wh = 25,000 kWh

2. How much energy (in kWh) is consumed by the 25 fluorescent 20 Watt bulbs during 10,000 hours of operation (their lifetime)? Energy consumed = = 25 bulbs 20 W/bulb 10,000 h = 5,000,000 Wh = 5,000 kWh

3. How much energy did we save by replacing the 25 incandescent bulbs by 25 fluorescent bulbs? Energy Saved = = 25,000 kWh – 5,000 kWh = 20,000 kWh

4. How much money is saved, assuming that the cost of electricity equals $0.10/kWh? Money Saved = = 20,000 kWh $0.10/kWh = $2,000

5. What is the payback period assuming the fluorescent bulbs cost $4 each? (Neglect the cost of the incandescent bulbs.) Cost = $4/bulb 25 bulbs = $100 Savings= $2,000 / 10,000 h of use = $0.20 /hour Payback = Cost / Savings = $100 / $0.20 / hour = 500 hours of use (Assuming lights are used 10 hours per day, this is about 2 months. After this, the bulbs save $1,900 over their 3 year lifetime.)