Another pitfall with comparing lamp source efficiency is the entire system’s efficacy.
With incandescent and halogen sources, the wattage on the label is pretty much what you get.
- Technical info, you don’t have to read or understand this: the “what you see is what you get” assumes that the voltage matches. That is, a normal 100 watt light bulb will consume 100 watts at normal household voltage of around 120V. If the voltage is increased, the wattage increases, and if the voltage is decreased the watts consumed also decreases. Some manufacturers use this effect to create longer life and energy savings bulbs; for example, GE sells a 100 watt 130 volt lamp (3,000 hour life) which is labeled for 120 volt use as using 89 watts (8,300 hour life). Of course, the light output also decreases, and it decreases faster than the energy savings: losing 25% of the light output while only saving 11% of the electricity consumption.
Discharge sources, like fluorescent, require some form of control gear to regulate the electricity being consumed. The wattage label on the bulb is not the wattage which will actually be used in the system because the lamps can’t regulate their energy consumption themselves. They require a ballast to do that for them.
Since the ballast is what actually controls the wattage consumption, you have to calculate the efficiency of the system using both the lamp (for the lumen output) and the ballast. Professionals use the term efficacy instead of efficiency to remind themselves that they are dealing with the multipart system rather than just a lamp. So if you compare the efficiency of an incandescent bulb to a fluorescent replacement, you have to make the comparison against the system’s efficacy, not the lamp efficiency.
For the self-ballasted compact fluorescents you pick up at Wal-Mart the comparison is still fairly easy, since the ballast is included as a part of the lamp and the wattage listed on the box is for the ballast. In that regard, you can compare a regular light bulb off the shelf with the CFL next to it. However, if you live somewhere like California, where they have outlawed the use of medium screw based sockets in new construction (emphasis on new), you can’t use a self-ballasted CFL anymore.
Instead, you have to look at the efficacy of the lamp/ballast combination. Different ballasts will have all kinds of different ratings and properties that make reviewing them mind-numbing for the average person who just wants a light bulb. The take away is that the lamp ballast efficacy will be different than the lamp source efficiency. Usually it is less, although sometimes a system can have better efficacy than its lamp’s efficiency.
- Technical info, you don’t have to read or understand this: Ballasts affect the lumen output out the lamps they control separate from the age issue I discussed previously. The affect is known as the “ballast factor,” which is the ratio of what the ballast does to the lamp it is powering versus the fictitious “perfect laboratory ballast” which is used to give the lumens shown on the lamp box. The number is a percentage (real output divided by “perfect” output) and written to two decimals (0.77 or 1.53). You will see ballast factors greater than 100%, accompanied by a wattage higher than the wattage listed for the lamp alone. To determine actual lumen output, you have to multiple the listed lumens by the ballast factor. This new lumen value is what you use to determine the efficacy of the system, along with the ballast’s wattage for that lamp. One ballast may have different ballast factors for different lamps, which will be listed. For easy comparison of ballast factors among different ballasts, there is also a “ballast efficacy factor,” which is the ballast factor divided by the input watts. The higher this BEF the more efficient the lamp/ballast combination, so you can compare ballasts for the same lamp that may have different wattage usage.
An example to demonstrate that you have to look at both the lamp and ballast:
I’m using a 4′ long linear T8 lamp, like you see in a 2′x4′ office fixture that is recessed in a grid ceiling made up of 2′x2′ tiles. To be specific, it is a GE F32T8/SPX35/ECO, which provides 2800 (mean) lumens for 32 watts. My first ballast option is going to be a GE “Residential Grade ProLine” ballast (GE232-120-RES). The lumen output for one lamp is 2,772 (mean lamp lumen times the ballast factor). The system wattage is 32, so the efficacy of the system is 86.6 LPW. I then substitute the “Residential Grade” for a regular “ProLine” ballast (GE232-120-N). The lumen output for one lamp is now 2,632. The system wattage is 36, so the efficacy is now 73.1 LPW. I want to try one more ballast and I choose the “UltraMax Instant Start Multi-Voltage High-Efficiency” because the name sounds cool and highly efficient (GE132MAX-L/ULTRA) The lumen output is now 2,156. The system wattage is 25, so the efficacy is 86.2 LPW.
To choose my ballast, and because I don’t care about how much each one costs for this example, I discount the “ProLine” ballast since it gives me similar output to the “Residential Grade” ballast with less efficacy. Now, I have to choose between the “Residential Grade ProLine” and the “UltraMax.” They have similar efficacies, so I have to choose which I want based on the lumen output. What I haven’t told you yet is that I’m replacing a surface mounted fixture in my home that uses two 75 watt bulbs. The lumen output of the existing fixture that I am trying to match is 2,340, which falls between the 2,772 of the “Residential Grade” ballast and the 2,156 of the “UltraMax.” Rather than decide based on the efficacy or the wattage of each individual ballast I have to select the ballast based on the light output. I decide that I always though the room was a little bit too dark anyway and so I choose the “Residential Grade” ballast. Now, I’ve got a touch more light than before while saving about 75% of the energy, dropping from 150 watts down to just 32!
PS: I’ve written this using fluorescent lighting as the examples, but the same is true for all the gas discharge sources, like metal-halide, high-pressure sodium, etc. High-pressure sodium is an especially efficient light source, although it suffers from all those color temperature and color rendition problems discussed in previous posts.
