I'm looking at replacing 175 W mercury halide bulbs used outside with 28 W LED (mogul bases).  The LED bulb manufacturer claims light output is equivalent

This looks like a 'no brainer:'  It's over 6X more power efficient.  And when you count the ballast in for the MH bulbs, that's another 15% watts that's avoided.  And the 28 W LED bulbs have a 50,000 hr lifetime vs 10,000 for the MH.  And for outdoor lighting, I wouldn't think CRI is a major issue.

So what am I missing?  Is it truly an obvious situation or are there 'gotchas' that end up making the switch to LED more expensive than say T8 linear fluorescent for a high output outside setting?

Many thanks!

Best regards,

Bill

No way is the light output equivalent. It *might* have same output as a 28W mercury halide lamp.


http://en.wikipedia.org/wiki/Luminous_efficacy

Yup- the marketing hype has far outstripped reality in LED performance. Unless you have the worlds least-efficient 175W mercury systems out there (underperforming the manufacturer's spec) a 28W LED won't have anywhere near the same output.

The advantage LEDs have over fluorescent and mercury is instant start even at cold temps, making them more suitable for motion-control sensor and (and often, dimmer control.) Even at lower lumens/watt, if the duty cycle is smaller or the output is modulated they can consume less average power. The other advantage is longevity. Although lumen output isn't maintained, if properly designed they'll last pretty much forever- a 50,000 hour LED will very likely be putting out SOME light at 100,000 hours.

Dana1, I have continued looking at this, and can't figure out how to make the numbers work.  As you say, regardless of what the definition of light output is (lumens vs candlepower vs 'light the hits the deck') there just doesn't appear to be any way a 175 W MH fixture can be approximated by a 28 W solid state fixture.  Application is a parking garage where the lights are on 24 x 7.

Many thanks for the comment.

Best regards,

Bill

Posted By a0128958 on 18 Mar 2010 03:23 PM

Dana1, I have continued looking at this, and can't figure out how to make the numbers work.  As you say, regardless of what the definition of light output is (lumens vs candlepower vs 'light the hits the deck') there just doesn't appear to be any way a 175 W MH fixture can be approximated by a 28 W solid state fixture.  Application is a parking garage where the lights are on 24 x 7.

Many thanks for the comment.

Best regards,

Bill

It's hard to beat the ~90-120 lumens/watt of T8 or T5 linears in price/performance in this type of app when you consider the efficacy of the higher color-rendering you get out of them compared to HIDs of higher lumens/watt with lousy CRIs.

eg: Low pressure sodium can run ~150lm/w give or take but in a color spectrum that has poor contrast for human eye perception. It's fine for the black & white security cameras, but it's harder for humans to see with.  With broader/better spectrum of fluorescent's humans can find their car by the color more readily, find the keys, read the parking notice under the windshield or identify the attacking criminal better at lower light levels than with high efficiency/low light quality HIDs. 

Metal halides can have equal or better efficacy to fluorescents at similar lumen-efficiencies, but have a huge glare-factor- they're better for high-bay apps like a parking LOT, as opposed to a parking GARAGE.