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SOLAR PATHWAY LIGHTS WITH LED LAMPS
SOLAR GARDEN LIGHTS
SOLAR TRANSFERMER

 

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Operating Data and the Economics of Different Lamps*

 

Assume: 4100 hours of use per year (average night time hours, dusk to dawn) 8c per KWH (typical average cost per kilowatt-hour, the power rate)

Low Pressure Sodium

 

 

180W

 

135W

 

90W

 

 55W

 

35W

 

18W

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Initial Lumens 

 

33000

 

22500

 

13500

 

8000

 

4800

 

1800

 

Mean Lumens

 

33000

 

22500

 

13500

 

8000

 

4800

 

1800

 

Lamp Wattage 

 

180

 

135

 

90

 

 55

 

35

 

 18

 

Circuit Wattage

 

220

 

180

 

125

 

80

 

60

 

30

 

Initial Lum/watt

 

150

 

125

 

108

 

100

 

80

 

60

 

Mean Lum/watt

 

150

 

125

 

108

 

100

 

80

 

60

 

Annual KWH Use

 

 902

 

738

 

513 

 

328

 

246

 

123

 

Annual per Cost 

 

$72.16

 

$59.04

 

$41.04

 

$26.24

 

$19.68

 

$9.84

 

High Pressure Sodium

 

 

400W

 

250W

 

200W

 

150W

 

100W

 

70W

 

 50W

 

35W

 

Initial Lumens

 

50000

 

28500

 

22000

 

16000

 

9500

 

6300

 

4000

 

2250

 

Mean Lumens

 

45000

 

25700

 

19800

 

14400

 

8550

 

5670

 

3600

 

2025

 

Lamp Wattage

 

400

 

250

 

200

 

150

 

100

 

70

 

50

 

35

 

Circuit Wattage

 

465

 

294

 

246

 

193

 

130

 

88

 

66

 

46

 

Initial Lum/watt

 

108

 

97

 

89

 

83

 

73

 

72

 

61

 

49

 

Mean Lum/watt 

 

97

 

87

 

80

 

75

 

66

 

64

 

55

 

44

 

Annual KWH Use

 

1907 

 

1205

 

1009

 

791

 

533

 

361

 

271

 

189

 

Annual per Cost

 

$152.56

 

$96.40

 

$80.72

 

$63.28

 

$42.64

 

$28.88

 

$21.68

 

$15.12

 

Metal Halide

 

 

1000W

 

400W

 

 250W

 

175W

 

150W

 

100W

 

70W

 

50W

 

32W

 

Initial Lumens

 

110000

 

36000

 

20500

 

16600

 

13000

 

9000

 

5500

 

3500

 

2500

 

Mean Lumens

 

88000

 

28800

 

12700

 

10350

 

8700

 

6400

 

4000

 

2500 

 

1900

 

Lamp Wattage

 

1000

 

400

 

250

 

175

 

150

 

100

 

70

 

50

 

32

 

Circuit Wattage

 

1070

 

456

 

295

 

215

 

184

 

115

 

88

 

62

 

43

 

Initial Lum/watt 

 

103

 

79

 

69

 

77

 

71

 

78

 

63

 

56

 

 58

 

Mean Lum/watt

 

82

 

63

 

58

 

48

 

47

 

56

 

45

 

40

 

44

 

Annual KWH Use

 

4387

 

1870

 

1210

 

882

 

754

 

472

 

361

 

254

 

176

 

Annual per Cost

 

$350.96

 

$149.60

 

$96.80

 

$70.56 

 

$60.32

 

$37.76

 

$28.88

 

$20.32

 

$14.08

 

Mercury Vapor and Incandescent *

 

 

1000W

 

700W

 

 400W

 

250W

 

175W

 

100W

 

 

 

150W*

 

100W*

 

Initial Lumens

 

55000

 

36400

 

20500

 

11850

 

7850

 

4100

 

 

 

2850

 

1710

 

Mean Lumens 

 

46200

 

29850

 

18570

 

10540 

 

7140

 

3230

 

 

 

2850

 

1710

 

Lamp Wattage

 

1000

 

700

 

400

 

250

 

175

 

100

 

 

 

150

 

100

 

Circuit Wattage

 

1090

 

765

 

455

 

285

 

205

 

135

 

 

 

150

 

100

 

Initial Lum/watt

 

50 

 

48

 

45

 

42 

 

38

 

30

 

 

 

19

 

17

 

Mean Lum/watt

 

42

 

39

 

41

 

37

 

35

 

24

 

 

 

19

 

17

 

Annual KWH Use

 

4469

 

3137

 

1866

 

1169

 

841

 

554

 

 

 

615

 

410

 

Annual per Cost

 

$357.52

 

$250.96

 

$149.28 

 

$93.52 

 

$67.28 

 

$44.32

 

 

 

$49.20 

 

$32.80

 

lined up at nearly equal lumen output, to show the relative energy & cost savings.

Definitions and Discussion Points

  1. The numbers in the preceding table are approximate. Lumen output depends on the bulb manufacturer and operating conditions. Circuit wattage depends on the ballast manufacturer.
  2. The numbers in the preceding table are for clear bulbs. Diffuse coated ("frosted") bulbs are available for most lamp types, and these will have a somewhat lower lumen output. Always use diffuse coated bulbs when the light source is directly visible from normal viewing angles to reduce glare. Use clear bulbs in fully shielded fixtures or when the fixture lens is diffuse or translucent.
  3. We use 4100 hours as typical of the annual operating time of a street light or any other fixture controlled by a photosensor that comes on at dusk and goes off at dawn. 4100 / 365 = 11.23 hours per night. A sampling of several cities indicates that 4100 hours is typical of the hours that their street lighting system is operating each year.
  4. The U.S.A. national average for electrical utility rates is close to 8 cents per kilowatt-hour. One can and should use a rate that is representative of local utility rates. The range is from a low of about 4 cents (wouldn't that be nice in your own area?!) to a high of 18 cents or more. Any spreadsheet program makes such comparisons easy. One should allow for future rate changes, which are most generally upwards.
  5. Kilowatt-hour (KWH) is a measure of the amount of energy used. Kilowatts measure power. A kilowatt is 1000 watts. A KWH is one kilowatt of power used for a duration of one hour.
  6. Initial lumens is a measure of how much light the lamp is emitting near the beginning of its life. Most high-efficiency light sources (except LPS) decline in light output with time. LPS has a lifetime of about four years, and HPS about five, while mercury vapor almost never "burns out"; it just keeps getting fainter and fainter. You can estimate the relative effects by looking at the row titled "mean lumens". This is the average output of the lamp during its usable lifetime.
  7. Mean lumens is a measure of how much light the lamp is putting out after about two or three years of usage. We a ssume a typical lifetime for the lamp, either due to burnout of the lamp or to group replacement. Many communities replace lamps after a specified interval, so as to minimize any outages due to lamp burnout. The cost of a lamp is much le ss than the cost of an accident or a lawsuit due to a lamp having burned out. The i ssue of half life and replacement strategy is complicated, and few agree on all aspects.
  8. Circuit wattage takes into account the other energy uses besides that of the lamp. The major energy lo ss occurs in the ballast, a unit needed to start and operate the lamp under conditions that it is designed for. There are many different kinds of ballasts, and what is good for one lamp or wattage is usually not good for another. LPS should be used with a ballast designed for efficient LPS use, for example. The ratio of lamp wattage to circuit wattage is not a constant, even for the same type of lamp. See the table for examples.
  9. All these entries have been taken from either lamp manufacturers' catalogs or actual operating experience in different communities. The figures given in the table are sort of an average of all that, and as such should be typical of what is being used in any specific location.
  10. Lumens/watt is a measure of operating efficiency: total amount of light from the lamp per power used.
  11. Annual KWH use is also a measure of operating efficiency, as it tells how much energy is used each year. Naturally, don't use more light than one needs (more light is not always better!) as that uses more energy.
  12. Typical wattages for major highways or streets would be 180 or 135 or 90 watt LPS, or 400 or 250 or 150 watt HPS, or 1000 or 400 or 250 watt Mercury Vapor. Typical values for residential streets might be 90 or 55 watt LPS, or 150 or 100 or 70 watt HPS, or 175 watt mercury vapor. Typical home security lighting might be 35 or 18 watt LPS, 70 or 50 or 35 watt HPS; please don't use mercury vapor, as it is not very efficient. Always use full-cutoff fixtures for all applications!
  13. Annual operating cost is another measure of operating efficiency, of course. It tells how much one must pay for energy usage in order to operate one given fixture for one year. In some cases, the cost of the fixture is le ss than the annual operating cost! Payback times when replacing inefficient fixtures with energy efficient fixtures can be very short. Quite often, a one-step-lower-wattage bulb (and ballast) can be used, resulting in lower operating costs.
  14. Of course, there are other costs for any given installation. Maintenance, lamp replacement, replacements due to accidents and breakages, depreciation, whatever. Generally these are "a wash" as all systems have similar costs.
  15. As you look at the table, be sure to notice the bulb wattages that give similar light output for different types of lamps. For example, 35 watt LPS, 70 watt HPS, 100 watt Metal Halide, or 175 watt Mercury Vapor give similar mean lumen outputs. Such comparisons can offer guidance as to the tremendous savings that can be obtained with more efficient light sources. Keep in mind, though, that an inefficient source used infrequently uses le ss energy than a highly efficient source that burns from dusk to dawn, 365 nights a year. Thus, an incandescent light that is activated by an outdoor occupancy sensor will usually have a lower operating cost than a dusk-to dawn HPS security light, for example.

There are other overall considerations as well. For example, not all fixtures are equally efficient at getting the light produced by the lamp out of the fixture and onto the area needing the light. One should always use efficient fixtures as well as efficient lamps. Many old fixtures are not efficient, as they were designed at a time when energy was cheap and efficiency was low on the priority list. For example, "globes" throw more than half their light output upwards. Today, there is no excuse to use any such inefficient fixtures. Please help stamp them out. Use efficient full-cutoff fixtures for all applications. Install as recommended, of course, to insure that the light output is used, not wasted producing glare and uplight.

 

 

* As per US electrical utility rates.

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