Ballast efficiency vs input watts

[Npstewart]

Hi everyone, I have kind of a stupid question that someone asked me and realized I wasn't really sure either.

If you have (3) 32w lamps powered with a ballast that is 90% efficient, what is the total input watts of the light fixture?

My thinking was 3*32w = 96w then because the ballast is 90% efficient, 1/0.9=1.11 so 96*1.11 =106.6 total watts.

That seems correct but then I started thinking that the efficiency is just telling you that of the 96w, only 90% will be used for the actual light and the other 10% is just lost as heat so the input power will still be 96w.

So basically my first assumption is only correct if you wanted to figure out how much power the light needs to get the full lighting output of the 96w but doesent really have anything to do with the actual input watts.

 

[steve66]

Hi everyone, I have kind of a stupid question that someone asked me and realized I wasn't really sure either.

If you have (3) 32w lamps powered with a ballast that is 90% efficient, what is the total input watts of the light fixture?

My thinking was 3*32w = 96w then because the ballast is 90% efficient, 1/0.9=1.11 so 96*1.11 =106.6 total watts.

That seems correct but then I started thinking that the efficiency is just telling you that of the 96w, only 90% will be used for the actual light and the other 10% is just lost as heat so the input power will still be 96w.

So basically my first assumption is only correct if you wanted to figure out how much power the light needs to get the full lighting output of the 96w but doesent really have anything to do with the actual input watts.

Most ballasts don't drive 32 watt lamps right at 32 watts. The ballast factor tells you how much power the ballast sends to the lamp. Common ballast factors are .88, .95, 1.00, and 1.15.

So to figure out the input power, you need to know the lamp wattage, the efficiency, and also the ballast factor.

 

[kwired]

I do not claim to be an expert in this area, but I would like to think the 32 watt rating is the designed rating of the lamp. It possibly could be driven by a lower value, but a higher value likely would result in shorter life. But then I could be totally wrong.

I think same goes for HID lamps or incandescent lamps. If you have a 400 watt HID lamp the lamp itself is likely going to use 400 watts, the ballast will add some power to the required input, and power factor will change actual volt-amps.

For incandescent lamps if it is rated 100 watts, 125 volts then it will use 100 watts (if supply is 125 volts) - there is no ballast to lose anything through is really the biggest difference as far as marked wattage vs input wattage for the "system".

I could be wrong but I kind of have a feeling there are regulations somewhere that will say the lamp is marked 32 watts then it must use 32 watts at least if all other tested conditions to come up with 32 watt rating are met.

Look at the old T12 lamps. They used to be 40 watt lamps long time ago. Changes in efficiency and other standards reduced the most common lamps to being 34 watts yet they ran on same ballast designed for 40 watt lamps. I would like to think they were that much more efficient, or there was less light output, or both. Look at marking on ballasts and they usually indicated a lower line amps for the 34 watt lamps then for a 40 watt lamp - which is logical.

 

[shamsdebout]

Very good question actually as it led me to some new information after a google search.

Where?s the Watts?: OSRAM SYLVANIA?s two-lamp FO32T8 electronic instant start ballast has an input of 59 watts. This number is comprised of lamp wattage plus ballast losses (the power that is dissipated as heat instead
of being converted to light). Each FO32T8 lamp is rated for 100% light output at 32 watts low frequency rapid start, but only 28 watts when operated on high frequency electronic instant start ballasts (due to the fact that lamps operate more efficiently at high frequency). Therefore, each of the two lamps operates at 28 X 0.90 ballast factor = 26 watts per lamp. The other seven watts in this system are the ballast losses.

Input Watts/ANSI Watts. Input watts published by ballast
manufacturers are the total watts consumed by both the ballast
and the lamps it operates. ANSI watts are the rating given for a
ballast measured under the strict testing procedures specified by
ANSI standards and are a dependable measure of this lamp/ballast
performance. Energy savings can be determined by comparing the
input watts of different lighting systems.
Input watts may be affected by tolerance build-up from the ballast,
lamp, input voltage and ambient temperature. The input watts