John,
Your model assumes greater efficiency gain than mine, thus requires fewer bulbs needed to achieve the same savings.
You're replacing 32W with 18W, a 45% efficiency gain.? A modern 32W T8 produces around 3000 lumens, almost 50% more than a 18W LED T8.? Your model also assume a loss of lumens, from 2450 to 2100 per tube.?
I held lumens constant, so I assumed the old tubes to be 25W, which has same lumen output as 18W LED, yielding a 28% efficiency gain.
LED T8 marketing often describe 18W LED T8 as "equivalent to 32W fluorescent" or "replaces 32W fluorescent".? ?I believe the basis of this claim is a notion of "effective illumination", assuming the directionality of LEDs yields less wasted lumens.? But in actual lumen output, a 18W LED is ~70% of a? T8 fluorescent.
The details make a non-trivial difference, for example the type and quality of reflectors, location of fixtures, etc.
A 30% efficiency gain per se seems attractive, but context matters a lot.? Obviously that's huge for 5 million square foot office tower.? For a small shop in cold climate (where one is using the waste heat), it might be barely measurable in reality.?
Btw, I am guessing Jason didn't charge himself for his labor.? If it takes half hour per fixture and labor is $100/hr and he had 30 fixtures, that's $1,500, moving the payback from 1 year to ~2.5 years.? ??
