Our Townhouse shares an underground garage with 7 other units. There are 12 8’ double lamp fluorescent fixtures with T12 60W bulbs in the common area. Half of them are on 24 hours a day (no switch except the breaker box), and the other half are on a timer. Currently, we leave the timed fixtures off, since there is more than enough light from the others. I would like to replace the fixtures that are always on with 4’ single lamp units using T8 32W bulbs, and use the timed lights to augment the light from dusk til, say, midnight, when it is likely to be needed.
My first question is more like verification that I am calculating kilowatt usage (and savings) correctly. I used (number of bulbs) x (rated wattage) x (number of hours on) x (cost per kilowatt). i.e. the existing fixture would be (2 x 60 x 8760), or approx. 1051 kW per year (times the cost per kW) The replacement single bulb would be (32 x 8760), or approx. 280 kW per year, right?
The reason I ask is because I was looking at specifications for fixtures, and a 4’ T8 dual lamp for $20 carried the notation that it was Energy Star compliant. A similar single bulb unit by the same manufacturer (for $30) specifically stated that it was NOT Energy Star compliant. This doesn’t make since, unless there is something else going on here that I am not aware of. I would still like to put in the single bulb fixtures, even though it is more expensive up front, since we recoup the additional cost in 3 or 4 months, but I want to make sure all of my assumptions are correct and I haven’t left out something critical that affects kilowatt usage (and Energy Star ratings).
I can’t help with the exact detail of your question, but with some experience with Energy Star standards…
Energy Star with lighting doesn’t necessarily look at electricity consumption alone. Light output of the fixture and distribution of the light output can also affect the rating, as well as the functional purpose of the light.
For example, a fixture used as a workbench shop light may not be considered Energy Star compliant, while one using the same tube for a kitchen counter could be compliant, even though both have exactly the same electrical output and efficacy. It gets more complicated when you get into CFL replacements for incandescents, where light distribution from the fixture or bulb opens up a whole zoo of light sub-categories.
If you are concerned purely about your power bill, I would think your power arithmetic would be sufficient regardless of whether it carries an Energy Star rating.
You appear to have forgotten the ballast factor. Fluorescent lights use a ballast to generate the high voltage required for their operation. The ballast factor is a multiplier on the rated wattage of the tube. If the ballast factor is less than 1, the lamp consumes less power than the rated watts. 0.8 is a common energy saving ballast for “super T8” fixtures. A 32 watt bulb used in a fixture with a 0.8 BF ballast only uses 25.6 watts. Ballast factors can be greater than 1 and result in more light from the same lamp.
I suspect this is what seperates the energy star from the non e-star fixture.
If the single bulb unit has a BF of 1.1 it would consume 35.2 watts.
This would still save power over the dual light unit, even if that unit has the 0.8 BF ballasts.
Yes, good point about the ballast, even more to try and be aware of.
This reflects a common complaint with the Energy Star branding with lighting especially, in that it makes informed choice at the retail purchase point rather difficult for the consumer.
A nitpick about your calculations: The cost you want is not a cost per kilowatt, but a cost per kilowatt-hour. The fact that electric costs are quoted in costs per kilowatt-hours is the reason why you needed to multiply by the number of hours, and not some other time unit.
That said, aside from your labeling, the calculation is set up correctly.