[...]Efficiency Comparison Pitfalls: Systems | Light Thoughts[...]…

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

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

Is there a way to be exact about the true wattage consumption?