Are my lights super amazing ridiculously efficient?!

[BatteryCharger]

I bought one of those Kill-a-Watt meters a few days ago. OMG - I've been testing everything! The "phantom" loads on things that are turned off are almost sickening.

Anyway, I have these T8 fluorescent light fixtures with Sylvania "Quicktronic" professional series electronic ballasts. Hooked up to the ballast are two standard 32 watt T8 tubes. (F32T8) Common sense tells me it should draw 64 watts, plus some overhead for the ballast...right?

Then how come it's only pulling 47 watts? Both tubes are lit up, and to my eye, as bright as they should be. The Kill-a-Watt shows power factor at 1.0 and it draws 47watts/47vA at 120 volts.

Is this ballst underpowering my tubes for some reason? I have several of these light fixtures and they all show exactly the same wattage.

 

[JohnR66]

Does the ballast show a ballast factor rating? A number of less than 1 will mean the bulbs are under-driven. This is not at all uncommon. If you are satified with the light output, be happy it draws so little power.

 

[jtr1962]

It sounds like your ballast is one of those "energy saver" ones which powers a tube at rough 70 to 75% of rated output. One thing to note is that T8 tubes have their lumens rated on magnetic ballasts, but when running on electronic ballasts they output roughly 10% at the same power consumption. Your ballast and tubes together consume 47 watt. Let's assume the ballast is roughly 90% efficient, so about 21 watts makes it to each tube. Tube output versus power is roughly linear, so that would be about 66% of rated output if this was a magnetic ballast. With an electronic ballast you'll get ~10% more, or approximately 72-73%. A standard non-energy saver ballast drives the tubes at usually 89%. Your eye isn't really going to be able to tell the difference here between 73% and 89%, so yes, the tubes will appear to be full brightness.

T8s are super efficient compared to old school T12s. I remember some shoplights with magnetic ballasts using around 100 watts for 2 F40T12 tubes. The 2xF32T8 shoplights I replaced them with actually give a little more light, and use only around 60 watts.

 

[BatteryCharger]

T8s are super efficient compared to old school T12s. I remember some shoplights with magnetic ballasts using around 100 watts for 2 F40T12 tubes.

Yeah, I have 20-something 8' T12s in my shop - 110w each plus magnetic ballast losses. I'm probably going to replace them all with T8.

I found the spec sheet for this ballast and it says it has a ballast factor of .88. Does that mean a given tube will output 88% of it's rated lumens? (and does that also mean the tube will last longer?)

 

[jtr1962]

I found the spec sheet for this ballast and it says it has a ballast factor of .88. Does that mean a given tube will output 88% of it's rated lumens? (and does that also mean the tube will last longer?)

That's exactly what it means. A BF of 0.88 of pretty much standard as far as ballasts go. Regarding lifetime, believe it or not underdrivng a fluorescent tube really doesn't prolong life by much. Lifetime is heavily dependent upon the number of starts rather than the drive level.

 

[SemiMan]

That's exactly what it means. A BF of 0.88 of pretty much standard as far as ballasts go. Regarding lifetime, believe it or not underdrivng a fluorescent tube really doesn't prolong life by much. Lifetime is heavily dependent upon the number of starts rather than the drive level.

I agree to a point about the lifetime being dependent on starts, but that is really a factor of the on/off cycles. If you are only turning them on/off once or twice a day and running them for say 4-8 hours at a time, then yes, running them at a lower level is going to significantly improve life. Modern bulb/ballast combinations can go through more start cycles then older bulb/ballast combinations.

Semiman