T8 Fluorescent LED Replacment Bulb Energy Consumption

[eviecurtains]

I thought I'd give some of these LED bulbs a try in a fluorescent shop fixture.
http://www.homedepot.com/p/Philips-4-ft-T8-17-Watt-Daylight-Linear-LED-Light-Bulb-456608/206278132
Observations:

• Energy consumption measured using a Kill-a-Watt:
  • Dual bulb, electronic ballast, fixture only: ~5 watts
  • Fixture with two 32 watt (rated) fluorescent bulbs: ~48 watts
  • Fixture with two 32 watt equivalent LED bulbs ~ 39 watts (~23% decrease)

• Anecdotally
  • The LEDs came on instantly in my cool garage (probably 50 degrees F)
  • The light output seemed higher (but the existing bulbs are a few years old)
  • No discernible audible noise but I don't hear higher frequencies

At $0.12/kw-hr the break-even point is about a year's full-time usage in a bulb rated to last 4 years. (time value of money not considered)
I plan to install 4 more in the kitchen today in a fixture that gets heavy use.

Edit: The 4 I installed in the kitchen seemed to have really brightened the place though its hard to be objective since I went with a 5000k replacement and am sure the former bulbs were of a lower color temp.

 

[eviecurtains]

Seem great, but two ware good to provides plenty lighting.

 

[semipro]

There is another reason a lower lumen output from a LED may be brighter than a higher lumen output from a T8, and that is the LED projects all of its light at a downward angle, no light is projected upward, so all the light goes to the surface being lighted, and especially not to the ceiling or the reflector above the tube. It may be that a lot of T8 light goes where light isn't needed, that is, wasted lumens.

Good catch and a caveat for those hoping to install these in fixtures where the bulb may not face where you want the light. The contact pins need to lie in a plane parallel to the ground if you want the light to shine straight down.
These types of bulb holders do this. According to comments some bulb holders hold the bulb at a strange angle.

 

[iamlucky13]

My thought too. How human perceive light (and perhaps process the related imagery) is interesting and complicated.
The bulbs I installed were of a higher color temp so that may explain a lot.

The color temp specs you listed were close enough they should hardly even be perceptible. The less "peaky" spectrum of the LED's might make a small difference, but it shouldn't be much.

It may be that a lot of T8 light goes where light isn't needed, that is, wasted lumens.

This can account for a little bit of it, but a decent white coating on the fixture should be reflecting about 80% of that back down, and since only half the light shines upwards, that means only ~10% actual loss. However, dust on the top of bulbs that have been installed for a while can make that number higher.

Also something to consider is lumen depreciation from the old T8 lamps. Brand new out of the box they will give you 2800 lumens. They will get less and less brighter until end of lamp life.

That possibility occurred to me and seems like it could be the most significant explanation.

End of lamp life, by the way, is considered to be when only 90% of the lamps are putting out at least 70% of their initial lumens (in other words, 10% dimmed more than 30%). 70% is where most people notice a difference in brightness without looking for it.