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BPM Criterion-Referenced Test #5 Answer Key

You have just completed an audit of a local store with 3,000 square feet of conditioned floor space. You found out that the store hours are Monday through Friday (52 weeks a year, or a total of 260 days per year), from 9am to 6pm. You found that there are one hundred (100) T-12 fluorescent tube lamps (assume they use 48 watts each including ballast consumption) that are in use during store hours. Light levels throughout the store were measured and found to be appropriate. Refer to the Lighting Information table below for lighting power use and cost data.

Lighting Information
Tube Type
Power Consumption

Replacement Cost

T-12 fluorescent tube w/ ballast 48 Watts each $5.00 each
T-8 fluorescent tube w/ ballast 34 Watts each $8.75 each

Based on your audit:

  1. Determine the annual energy use of the existing lighting system (in kWhr and $, assuming an electricity cost of $0.08/kWh).
    (48W*100)*(9hrs/day)*(260days/yr.)*(1kWhr/1,000Whr)=11,232 kWhr

    (11,232 kWhr)*($0.08/kWhr)=$898.56

  2. Determine the lighting index for the store in Watts/square foot.
    (48W*100)/(3,000 sq. ft.) = 1.6 W/sq. ft.

  3. Select replacement lighting and estimate annual energy use of the new system
    (in kWh and $, again assuming an electricity cost of $0.08/kWhr).

    T-8 cost is $8.75 each and energy use is 34W each.

    (34W*100)*(9hrs/day)*(260days/yr.)*(1kWhr/1,000Whr)=7,956 kWhr

    (7,956 kWhr)*($0.08/kWhr)=$636.48
  1. Estimate the cost of retrofitting the lighting system.
    ($8.75 ea.)*(100 units)=$875

  2. Estimate a simple payback for the retrofit.
    Simple payback = cost/savings. $875/($898.56/yr. - $636.48/yr.)=3.3 years

  3. Determine what the new lighting index would be for the store in Watts/square foot.
    (34W*100)/(3,000 sq. ft.) = 1.13 W/sq. ft.

  4. Assuming a 4-year simple payback is acceptable, is the lighting retrofit cost-effective? What do you recommend to the store owner?
    Yes, go ahead with the retrofit
    (the simple payback of 3.3 years is less than the given 4 year limit).

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