Looking for some 8' 4 bulb T8 fixtures for my garage. I looked at some at Home Depot and the ballast said 1.22 Amps. Amps X volts would give me about 146 watts. Bulb wattage equals 128 watts. I will assume the missing 18 watts is lost due to heat and inefficiency of the ballast.
Partially. But as has already been mentioned, that's not the whole story.
A different similiar fixture showed .97 amps. However .97 amps times 120 volts is about 116.4 watts, so am I correct in guessing the bulbs will be dimmer as the ballast wattage is less than the bulb wattage??
Well, not exactly. With only this much info to go on, I would indeed guess that the latter fixture
probably runs the tubes somewhat dimmer than the former fixture; but that's still only a guess, and it's NOT simply because the ballast current is lower.
The key determinative data point for how much light any given fluorescent tube will emit (when new) is the BALLAST FACTOR of the ballast used to drive that tube. And while there can sometimes be a rough correlation between the Ballast Factor and the ballast current, they are NOT the same thing. Nor is "Ballast Factor" the same thing as the "Power Factor" mentioned by "rockwithjason" and "Kevin C".
you have to remember that you are dealing with nominal ratings here. you can't take that information at face value.
Actually, you can,
IF it is the right information, taken from a reliable source. For example, those ballast currents mentioned by "Rich720'
ARE determininative for figuring out total circuit load, and thus the wiring and breaker sizes that will be needed (at least presuming that he got those numbers directly off the ballast labels, as opposed to the fixture packaging or similar).
power factor and ballast design play a large part in florescent lighting load. each lamp will have a lumin rating on it and that is much more useful for telling you how bright a lamp will be.
The tube's Lumen rating is important, of course; but it is still influenced by the Ballast Factor. And thus, the light output WILL vary somewhat (perhaps significantly) depending on what fixture any given tube is installed in. Published Lumen ratings for fluorescent tubes are based on an ANSI-standard reference ballast. See
http://www.lrc.rpi.edu/education/learning/terminology/ballastfactor.asp and
http://ateam.lbl.gov/Design-Guide/DGHtm/ballastfactor.htm for confirmation and further background.
Peak current is not at peak voltage... On ballasts that don't have power factor correction, the actual watts used cant be determined by multiplying voltage times the current.
Actually, that applies to ALL ballasts. PFC just (partially) tames the magnitude of the phase shifts.
To compare brightness you want to look at ballast factor. Ballast factor is how much light the ballast is designed to put out compared to a reference ballast.
BINGO!
You want to look at lamp lumens and CRI (color rendering index) to select the best lamps. The higher the CRI, the better, it means it's closer to daylight color.
Well... Not exactly.
It's more a measure of how well (or poorly) the lamp covers the whole visible spectrum, as opposed to having large peaks and valleys in its spectral output curve, which merely "average out" to a given overall color temperature. And FWIW,
ALL fluorescent lamps utterly **** in this respect, as compared to both natural daylight and many other types of artificial light.
Personally, I'd consider 4' long fixtures instead of 8' fixtures if you are relighting the entire garage.
On this, we definitely agree.
(The 8' fixture will work well over a workbench.)
This would largely depend on the size of the workbench, AND what sort of task lighting is also installed there..
What you really want is uniformity. No bright areas and dark areas around the garage.
Well, at least not in those parts of the garage where you're actually going to work.
You can do this by evenly spacing the lights around the garage.
Actually in a typical "home garage" you do NOT want all the general lighting lined up in in a neatly arranged (but simplistic) row/column grid over the entire space, such as is usually suggested by most "lighting calculators". That will lead to too much of the installed light being wasted / blocked by things like overhead doors and automobile roofs.
While you are doing this as a DIY project, if you had it engineered and professionally installed, you'd have to meet your local energy codes. For a "workshop" it is 1.59 Watts/sq-ft.
As a practical matter, the far more important thing for energy efficiency is to carefully arrange the switching of these lights into several different "banks", so as to provide:
A. - Some low-intensity "walk-through" lighting so that you do NOT have to turn on several hundred/thousand watts-worth of lighting fixtures just to go grab a screwdriver or bring in the groceries.
B. - As much granular control over the remainder of the remainder of the general lighting as reasonably possible, so that the lighting will remain as even and shadowless as possible when not all of the lamps are in use. Even when you are ostensibly "working" in the garage, you will often NOT need full brightness (or full brightness everywhere, in larger shops/garages). For example, changing wheels/tires does not require anywhere near the same level of illumination as fine assembly work, or paint matching, or any of several other things.
