Anton Lohuis : The cost of light and how to save on it to reduce your energy bill.

When we buy and use lighting we are actually buying light for a certain period of time. We " invest " in a light source that will last for a certain period and that will give us light radiation in a certain quantity of LUMENS for every Watt of electricity that we will feed it during its life.

There is an enormous difference in the cost of those lightsources according to their technology and there are also enormous differences in the cost of operating them. What we actually see is that very expensive light sources offer us the cheapest ways of getting the most lumens during a longer life time.

Let us compare the different technologies  and then find out what light produced by them will cost us.

Lamp	Efficiency (lumens per watt)			Lamp Life (in hours)
Reflector incandescent	7	-	19	2,000	-	5,000
Standard incandescent	8	-	24	750	-	2,500
Tungsten-Halogen	12	-	36	1,000	-	3,000
Mercury vapor	20	-	63	12,000	-	24,000
Fluorescent (tubular)	41	-	91	7,500	-	24,000
Compact fluorescent	50	-	83	7,500	-	10,000
Metal halide	56	-	125	10,000	-	20,000
High pressure sodium	61	-	140	12,000	-	24,000
Low pressure sodium	100	-	183	10,000	-	18,000

Light emitting diodes 1W leds                          20              -      40              80.000            -    100.000

Suppose we need to have a quantity of light equal to 1 million lumens during 1.000 hours. This can be realized in the following manner and at the following different costs:

A) By the use of incandescent lamps.

Most incandescent lamps offer about 12 lumens per Watt. A lamp of 100W will thus render 1200 lumens. Incandescent lamps last for 1.000 hours, so will be fully exhausted during our experiment. In our exeample we go for 1 million lumens, so we are going to need 1.000.000 : 1.200 = 833 lamps of 100W. Such a lamp in a famous brand will cost about Euro 0,35 so our INVESTMENT will be 833 x Euro 0,35 = Euro 292.

To run the lamps with electricity for 1.000 hours we have to light up 833 lamps x 100Watt = 83.300 Watts per hour x 1.000 makes 83,3 kwh x 1.000 = 83.300 kWh. The cost of 1 kWk nowadays is about Euro 0,15 so the total cost of electricity will be 83.300 x 0,15 = Euro 12.495.

Our total cost is thus 292 + 12.495 = 12.787 EURO

As you can see the investment in buying the lamps represents not even 3% of the total cost!

B) By the use of energy saving lamps

Energy saving lamps are bended fluorescent tubes with integral ballast. They offer about 60 lumens per Watt. Their lifespan of good quality lamps is about 8.000 hours. In our example we will use the lamps for 1.000 hours only, so we are consuming (depreciating) only 1/8 of their initial cost. A 20 Watt energy saving lamp will give us 20 x 60 = 1.200 lumens. In our example we need 1 million lumens so we are going to need 833 lamps to achieve that. Such lamps in famous brands will cost about 8 Euro per lamp making a total investment of 833 x 8 = 6.664 Euro. For our calculation we only use up 1/8 of this amount so the real investment for the 1.000 hours only is 6.664 : 8 =Euro 833.

To run the lamps with electricity for 1.000 hours we have to light up 833 lamps x 20Watt = 16.600 Watts per hour makes 16.6 kWh x 1.000 - 16.600 kWh. The cost of 1 kWh nowadays is about Euro 0,15 so the total cost of electricity will be 16.600 x 0,15 = Euro 2.490.

Our total cost is thus 833 + 2.490 =  3.323 EURO

As you can see the investment in buying the lamps represents 25% of the total cost!

C) By the use of halogen MR 16 lamps

A halogenlamp emits about 22 lumens per Watt. The usual type is in 50W. Their lifespan of good quality lamps is about 4.000 hours. In our example we will use the lamps for 1.000 hours only, so we are consuming (depreciating) only 1/4 of their initial cost. A 50 Watt lamp will give us 50 x 22 = 1.100 lumens, so we are going to need 909 lamps. Such lamps in famous brands will cost about 2 Euro per lamp making a total investment of 909 x 2 = 1.818 Euro. For our calculation we only use up 1/4 of this amount so the real investment for the 1.000 hours only is 1.818 : 4 = 454,50 Euro.

To run the lamps with electricity for 1.000 hours we have to light up 909 lamps x 50Watt = 18.180 Watts per hour makes 18.18 kWh x 1.000 - 18.180 kWh. The cost of 1 kWh nowadays is about Euro 0,15 so the total cost of electricity will be 18.180 x 0,15 = Euro 2.727.

Our total cost is thus 454,50 + 2.727 =  3.181,50 EURO

D) By the use of metal halide lamps

Metal halide lamps are very beautiful lamps providing light close to daylight and very good colour rendering. They emit about 90 lumens per Watt. A lamp of 250W will thus provide us with 22.500 lumens. In our example we need 1 million lumens so we are going to need 45 lamps. These lamps in a famous brand will cost bout Euro 25 per piece leading to an investment of Euro 1.125. However, such lamps have a lifetime of about 10.000 hours, so we only consume 1/10 of its value during the first 1.000 hours so the real investment is only Euro 112,50.

To run the lamps with electricity for 1.000 hours we have to light up 45 lamps x 250W = 11.250Watts per hour makes 11.25kWh x 1.000 = 11.250 kWh. The cost of 1 kWh nowadays is about Euro 0,15 so the total cost of electricity will be 11.250 x 0,15 = Euro 1.687,50

Our total cost is thus 112,50 + 1687,50 = 1.800 EURO

E) By the use of sodium lamps

these sodium lamps are not suitable for normal lighting but they function well in glasshouses and for road lighting. Their colour rendering is very poor. They emit about 125 lumens per Watt. A lamp of 400W will thus provide us with 50.000 lumens.  In our example we need 1 million lumens so we are going to need 20 lamps. These lamps in a famous brand will cost about 30 Euro per piece leading to an investment of Euro 600. Since the life of the lamps is 16.000 hours we will use up only 1/16 part of the value so the real investment is only Euro 37.50

To run the lamps with electricity for 1.000 hours we need 20 x 400W = 8.000 Watts per hour makes 8.000kWh x 1.000 = 8.000 kWh. The cost of 1 kWh nowadays is about Euro 0,15 so the total cost of electricity will be 8.000 x 0,15 = Euro 1.200

our total cost is thus 37,50 + 1.200 = 1.237,50 EURO

E) By the use of light emitting diodes

In the very near future leds will also be used for lighting. They already perform well for signal lights and even in traffic lights. Early 2004 1W leds will be on the market providing daylight colour with 30 lumens per Watt.

In the initial stage the cost will be high and leds used in clusters will cost about 5 Euro per led used. The almost endless life of 100.000 hours is of course the main issue.

In our example we are going to need 33.333 keds that will give us the 30 lumens per piece to get to our 1 million lumens. The initial investment to purcahse these is 33.333 x Euro 5 = Euro 166.665.

In our example for 1.000 hours we only use 1/1000 part of our investment with a cost of Euro 1.667

To run the leds with electricity for 1.000 hours we need 33.333 x 1W = 33.330 Watts per hour makes 33.330kWh x 1.000 = 33.330 kWh. The cost of 1 kWh nowadays is about Euro 0,15 so the total cost of electricity will be 33.330 x 0,15 = Euro 4.999

Our total cost is thus 1.667 + 4.999 = 6.666 EURO

Conclusion: Even when leds cost 5 Euro per piece in early 2004 they will already be an interesting alternative to those using incandescent lamps and never wish to relamp again. We expect that within a couple of years cost will come down to less than 1 Euro per piece making it much more attractive to wider applications.

Not considering the leds we recommend to use only fluorescent tubes (much more economical than energy saving lamps) and metal halide lamps for most lighting applications.

Anton Lohuis