I want to make a comparative study of T12 40W and T8 led type tubes.
I just want to ask what is the additional ballast wattage should I add to get the total
Wattage of the system (lamp wattage + ballast wattage).

Do led tubes require ballasts?

Total wattage = lamp wattage * 1/(efficiency of power supply)

Light Guide: Fluorescent Ballasts

Efficiency can range from 70% to >95%

T8 LED Florescents

No Ballast, No starter

T8 LED Fluorescent Lamp, T8 Fluorescent Replacement, LED T8 Tube---www.edi-led.com

With LED's light output decreases somewhat over time.

So efficiency of power supply is nearly 100%

Now you get in to color perception, color temperature and spectrum and a better number Lumens/Watt.

Thanks everybody.. yes T8 led tubes has no ballast and starter.
What I need to do is to get the savings using T8 led tubes instead of using T8 traditional light source.
As I understand on computing it.. total wattage of Traditional tubes is equal to lamp wattage plus the ballast wattage. And I don't know what is the percentage of ballast do I need to add

base on the formula of keep it simple..

Example:
given :40 W tubes
Efficieny: 80%

Total wattage = 40W * 1/ .8 = 50 Watts..

Am I correct?

Yes

Let's revisit agan:

Lamp specs:

T40 40 W, 1000 Lumens
ballast 80% efficient
1000 lumens/(40 Watts * 1 /0.8) = 20 Lumens/Watt
Uses 50 Watts (40 * 1/0.8) to produce 1000 Lumens of light

T40 40 W 1000 lumens
1000 lumens/(40 Watts * 1/1) = 25 Lumens/Watt
Uses 40 Watts (40 *1) to produce 1000 Lumens of Light

So, in the 1st case you get less light output per energy used because of the efficiency of the ballast.

A Lumen is a measure of the light output.

The ballast itselt comes in various efficiencies most likely depending on when it was made. The efficiiency number should be written on the ballast or the manufacturer's specs especially for electronic ballasts.

Ballasts come in magnetic or electronic. The electronic ballasts will make the tube last longer. See if there is an efficiency number for a magnetic ballast in the reference otherwise use about 98% or 0.98.

The LED lamps probably won't burn out, but there intensity reduces over time. You might also find a lifetime spec of the bulbs too.

So cost of ownership depends on a lot of factors:

Cost of tube
Lifetime of tube
How efficient the tube is producing light
How efficient the power supply is supplying the tube

So the number you probably need is:

Initial cost + (Lumens per Watt) per (hr of tube life)

Where Lumens/Watt = Lumens/(Tube Wattage * 1/ efficiency)

This would assume that the electric cost is the same in both cases.

Got it?

Got it!
Well explained... thank you very much for your support guys...

My numbers aren't quite right for the cost of ownership because, I think they would have to be normalized to something like 1 yr, 2yr, 3yr, 4yr and 5yr and include a particular electric rate.

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