As I was finishing up my previous post about efficiency pitfalls, I realized that I had started an example which could be useful in and of itself. So I’m going to take it a step further.
Situation Summary: I have an existing fixture in my home that I want to make more efficient. It is a surface mount that looks like a glowing bowl stuck on the ceiling. It uses two 75 watt bulbs. The light in the room is more or less acceptable, although it is a little dim and I would like it to be slightly brighter if possible. Looking up the information about the existing bulbs, I see that the total lumen output of both bulbs is 2,340, so I know I want to at least match that and if possible increase it slightly.
I could have just substituted my 75 watt incandescent bulbs for 20 watt self-ballasted CFL bulbs, based on the label on the box that says to replace my 75 watt bulbs with the 20 watt CFLs. The energy savings is good, since now my fixture only uses about 40 watts instead of 150, meaning I’m saving about 73%. However, the mean lumens of those CFLs are only 965, so the new total would only be 1,930. Since I thought the room was already a bit too dark, going from 2,340 lumens to only 1,950 is going to make me unhappy over the long term, and I run the risk of someday switching back to the incandescents. (Note, the initial lumens of this bulb are listed as 1,150, which brings my total up to 2,300 and almost a perfect match to my existing. However, that is initial lumens and won’t last for the lifetime of the bulbs. They will continue to depreciate and my room will get darker and darker, so it is an unfair comparison to use the initial lumens.)
Fearing the decreased light output, I buy two 26 watt self-ballasted CFLs because I see that the mean lumens for those are 1,365, meaning my new arrangement will provide me 2,730 lumens! I’d be saving about 65% energy by using 52 watts, but I discover that the 26 watts lamps are bigger than my existing incandescents and they won’t fit in my fixture. The bowl hits the ends of the lamps preventing me from reattaching it.
Now I recall reading my own post from yesterday. I go to my local electrical supplier and buy a replacement fixture for the existing. Based on my post, I’m now using a long “blob” instead of a bowl on the ceiling. I get 2,772 lumens, so I am very happy with the new lighting level in the room, and I’m only using 32 watts. That’s a savings of about 75%, better than the savings from those CFLs! Granted, I’ve now bought a new fixture as well as lamp, and I had to be very careful about my lamp and ballast choice, but I’m much happier with the final result.
Of course, this just shows that in order to make the best choice you need to have some knowledge, but that’s what this blog is for. Feel free to send me questions or reply to my postings to get more information.
Tags: complaints, fluorescents, lamp life, light bulbs, tips
I am a little confused about info i have been told by several lighting product suppliers. They each say if I take the stated wattage of a T8 bulb (32) and times it by the stated ballast factor on the ballast, it will tell me the true wattage I will be paying the utility company for.
When I check the stated input watts of ballast vendor sales sheets, using the stated 32 watt T8, it is never the same! 32w x .88 = 28.16
If there are 4 bulbs it is a huge difference! If I am trying to figure this out for a small grocery store with 100 fixtures…
Is there a way to be exact about the true wattage consumption?
I am a little confused about info i have been told by several lighting product suppliers. They each say if I take the stated wattage of a T8 bulb (32) and times it by the stated ballast factor on the ballast, it will tell me the true wattage I will be paying the utility company for.
When I check the stated input watts of ballast vendor sales sheets, using the stated 32 watt T8, it is never the same! 32w x .88 = 28.16 Not 30w as stated on the ballast as “input power” or “input watts”.
If there are 4 bulbs it is a huge difference!
Fulham ballast states:
Model No. WHCG1-120-T8-IS
Input Power: 30w (with T8 32w lamp)
Is there a way to be exact about the true wattage consumption?
It is actually very difficult to calculate short of metering just the lighting circuits.
The suggestion offered by the suppliers is a typical shorthand used in the industry, although not technically accurate because the ballast factor actually deals with light output instead of wattage consumption. However, since light output and wattage consumption are related it works as a rough approximation.
A better method is to look at the ballast specifications for the “input wattage” or “system wattage.” This will give you the watts consumed for the specific number of lamps the ballast is supposed to control (given a particular voltage). Note that if the ballast can control more than one lamp quantity (one or two lamps), the relationship between the numbers might not make sense. For example, the system wattage for one 32 watt lamp might be 36 watts, but the system wattage for two 32 watt lamps might only be 57 watts.
You confusion may also come from the fact that the ballast factor is different for each ballast based on type and manufacturer. Not all T8 ballasts are the same. Also, there is no ballast that is perfect.
Remember, you can reduce actual wattage consumption by using a ballast with a lower ballast factor (for example, a ballast with a BF of 0.77 might consume 25 watts whereas a ballast with a BF of 0.87 might consume 28 watts), however, a similar decrease in light output is also going to occur.
If you are trying to determine your existing energy consumption it might involve looking at each ballast installed right now. If you do the maintenance in-house and have one standard ballast then you can use that as a reference. If you have outside electricians installing ballasts one by one as they burn out then you probably have a random collection of whatever was on the truck at the time of replacement, so figuring out what you have is going to be more difficult.
A more simple option may be a call to the power company to see if they have any energy conservation programs. They might have an option to install a temporary meter on your lighting circuits to determine your usage.
Yes, the whole thing is very difficult and messy. That’s why lighting consultants and electrical engineers have jobs. It is not cut and dry. If you need more help go to my regular website (msklighting.com) and send me an email.
Sincerely,
Mark Ketteran
I started responding to your reply before your clarification.
Quick answer: use the input power.
(The ballast itself uses some of the power, which has to be added to the power being sent by the ballast to the lamps. Electronic ballasts are typically more efficient than electromagnetic, aka magnetic, ballasts. Also, electronic ballast have less noise (that humming sound) than magnetic.)
Sincerely,
Mark Ketteran