Hi all,

Whenever you buy that energy saving lamp, you always see something like “80% saving in energy” on the packaging. I bet not many people know what this means, or how it was arrived at and how it applies in real life situation.

This is how it is arrived at:

For simplicity’s sake I will use the Osram Tungsten Incandescent Lamp (TIL) and Osram Compact Fluorescent Lamp (CFL) as the basis of my analysis…

The lamps lumen output for a 100 watts incandescent lamp is 1450 lumens (lm)…

The Luminous efficacy of the lamp is therefore 1450/100 = 14.50 Lumens per Watt (lm/w)

Compact fluorescent lamps have an average of luminous efficacy of about 70.0 lm/w…

A compact fluorescent lamp with an equivalent illumination (light output) level equal to that of a 100 watts incandescent lamp must give 1450 lumens.

Therefore the output power rating of the CFL is 1450/70 = 20.7 watts = 21Watts

We can therefore comfortably say 100watts incandescent lamp will give the same amount of light in a given horizontal plane as the 21watts compact fluorescent lamp under identical conditions…

Next:

The life of a compact fluorescent lamp is 15,000 hours while that of a tungsten filament lamp is 2,000 hours…

Power drawn by the compact fluorescent lamp is 21 Watts. Add 10% (2.1 Watts) losses in the electronic control gear within the lamp. Please note here that there is another loss (about 2% or 0.42 watts) due to increased current as a result of power factor being less than 1.0, which does not show itself in the CFL, but rather in the external wiring…

Total power consumed in the compact fluorescent circuit is therefore 21W + 2.1W + 0.42W = 23.52W.

Total power consumed over the entire lifespan of the compact fluorescent lamp = 23.52W x 15,000 watthours (wh) = 352,800 wh = 352,800/1000 Kwh = 352.8Kwh

Total power consumed by the tungsten filament lamp over a duration equivalent to the lifespan of the compact fluorescent lamp which is 15,000hours = 100W x 15,000wh = 1,500,000wh = 1,500,000/1000Kwh = 1,500Kwh

So, if you used tungsten filament lamp for 15,000hours you would use 1,500Kwh while using the compact fluorescent lamp over the same period of time, you would use 352.8 Kwh.

Therefore energy saved in preferring compact fluorescent lamp to incandescent over a duration of 1 lifespan of cfl (15,000 hours) is 1500kwh – 352.8Kwh = 1,147.2Kwh

Energy saved as a percentage is 1,147.2Kwh/1,500Kwh x 100% = 76.5%


The question now is, does this also translate into 76.5% cash saved?...Let us see…

(i) Power consumed by the compact fluorescent lamp in 15,000hours is 352.8Kwh…

As per KPLCs prevailing tariffs, total charges (with cfl) = 50Kwh x ksh 2.00 + (352.8 – 50)Kwh x 8.10 = ksh 2,452.68

Cost of a 21W compact fluorescent lamp is ksh 350.00

Total cost of the energy usage for 15,000 hours using CFL = ksh 2,452.68 + ksh 350.00 = ksh 2,802.68

(ii) Power consumed by the tungsten filament lamp in 15,000 hours is 1,500Kwh

As per KPLCs prevailing tariffs, total charges (with til) = 50Kwh x ksh 2.00 + (1500 – 50)Kwh x 8.10 = ksh 11,745.00

The life of a 100 W incandescent lamp is 2000 hours and therefore for 15,000 hours you will need 15,000/2,000 = 7.5 lamps = 8 lamps… cost of one 100 W incandescent lamp is ksh 60.00. Total cost of 8 TILs is ksh 60.00 x 8 = kshs 480.00.

Total cost of energy usage for 15,000 hours using TIL = kshs 11,745.00 + ksh 480.00 = kshs 12,225.00

(iii) Money saved on preferring the CFL to TIL is kshs 12,225.00 – ksh 2,802.68 = kshs 9,422.32

Money saved as a percentage is kshs 9,422.32/kshs 12,225.00 x 100% = 77.1%

@ Impunity and @ Limanika take note...

@kaigangio

There you pinned it right!! Energy saving CFL is the way to go.

jeez you cannot read the figures or what?

On light note could @ Kaigangio explain what is this animal called "lumen". It might help us to understand the basics.

Lumen = a measure of the total "amount" of visible light emitted by a source, its measured in units called "candela".

The candela is the SI base unit of luminous intensity.

Lumen, X-men, hymen...is there an English speaker in the house?

Hi guys

Seems there might be a problem getting the real definitions and meanings of the photometric quantities and parameters on the subject of light...

Let us start from here:

The study of light, its generation, propagation/transmission, manipulation, utilisation, properties and behaviour is known as Optics. This is one of the science subject studied in great detail in physics.

Photometry is the actual measurement of visible light in terms of its perceived nature/ brightness to human eye and vision....

The commonest photometric quantities relevant to the various photometric computations and designs are:

Photometric quantity.............SI Unit............Unit Symbol

1. Luminous Energy......Lumen Second.......lm-s

2. luminous Flux.......Lumen (cd-sr)......lm

3. Luminous Intensity...Candela (lm/sr)....cd

4. Luminance...........Candela/m2........cd/m2

5. Illuminance.........Lux (lm/m2)........lx

6. Luminous Emittance..Lux (lm/m2)........lx

7. Luminous Exposure...Lux-Second........lx-s

8. Luminous Energy density...Lumen Sec/m3...lm-s/m3

9. Luminous Efficacy...Lumen/Watt........lm/W

I bet upto here it is so far so good...

Great Bw. Landlord. But this had not been invented during out time of Eng.,Lit.,His & GP.

Next mbuzi you are the guest speaker.

So far so good, but would you kindly explain to us the actual meaning of those photometric quantities. It may help us in appreciating the information given in the lamps packaging.