Figure 2. A simple tool for calculating lamp burn time.

Using a spreadsheet program is one of the best ways that energy managers can use computers in their day-to-day work. Microsoft's Excel is the most common of these software packages. Spreadsheets perform repetitive calculations quickly and accurately, and thus are ideal for ensuring that proposed lighting designs meet various energy and customer requirements.

Predicting light levels or determining the number of luminaires required using simple zonal cavity calculations can be done using an easy to create spreadsheet template. The spreadsheet makes use of formulae from the Illuminating Engineering Society of North America (IESNA).

Average initial illumination (before normal depreciation factors begin to reduce the light level) can be calculated using equation 1, which is derived from the lumen method, and so is basically the mathematical definition of the footcandle.

Figure 3. A spreadsheet program automates the process of comparing lamps.

(Eq 1) (fc) = [luminaires (no.) x lamps (no.)/luminaire x lm/lamp x CU]÷area

where CU = coefficient of utilitization and area is given in square feet (ft2)

Equation 1 predicts the light level (in fc) for a given number of luminaires, number of lamps per luminaire, the initial lumen rating of the lamps, and the coefficient of utilization for the luminaire in the space it is lighting. Rearranging the equation provides a method to determine the number of luminaires required to provide a particular initial light level (fc).

(Eq 2) Luminaires (no.) = [fc x area]÷[lamps (no.)/luminaire x lm/ lamp x CU]

CalcuLite (www.screenmaker.com/ccl5.htm) or Simply Indoor (www.lighting-technologies.com) provide better looking screens and reports. Older versions of the CalcuLite required users to look up CU values from a table, but newer versions accept IESNA photometric data. Figure 1 shows CalcuLite's entry screen.

Figure 2 is a simple spreadsheet that I use to obtain annual lighting system burn hours. The example is for a manufacturing operation with different schedules for the factory and office lighting systems and with a busy season from November to April.

A spreadsheet can compare the performance of two light sources. I use a simple spreadsheet to analyze the savings expected when a compact fluorescent lamp replaces an incandescent lamp in a commercial application. The spreadsheet incorporates the three components of operating cost, energy, labor, and lamp price.

The spreadsheet can be duplicated using figure 3, which incorporates formulae from the IESNA Lighting Handbook. The key to lamp and maintenance costs is the formulae for the number of lamps per year, which uses the IESNA standard method. Cell references are by column (letter) and row (number) coordinates of the cells. Where there is only a cell coordinate shown (such as D8), the user must enter data. Manufacturers provide these data in catalogs and websites. Other information is available on project documents.

More complicated comparisons are easy to put into spreadsheet format.

Users who become skilled at creating and using computer spreadsheets can reduce the time spent on lighting and savings calculations in their everyday work.

Test Your Knowledge

1. The three components of lighting O&M cost include:

A. maintenance labor

B. lamp cost

C. energy cost

D. all of the above

2. The majority of lighting O&M cost is energy.

o true

o false

3. The advantages of using computer spreadsheets for lighting calculations include:

A. results look better

B. repetitive calculations can be done simply and accurately

C. neither a or b

4. Data for illuminance calculations are found in:

A. lamp and fixture catalogs

B. manufacturer's websites

C. either a or b

5. The average number of lamps used annually for a lighting system cannot be calculated using a spreadsheet.

o true

o false

Answers

1. The three components of lighting O&M cost include:

D. all of the above

2. The majority of lighting O&M cost is energy.

true

3. The advantages of using computer spreadsheets for lighting calculations include:

B. repetitive calculations can be done simply and accurately

4. Data for illuminance calculations are found in:

C. either a or b

5. The average number of lamps used annually for a lighting system cannot be calculated using a spreadsheet.

false