How do I calculate how the wattage required to run a 2' x 4' (4-4' light tubes) fluorescent fixture (mounted in a drop ceiling, if it matters)? I also need to calculate the wattage for several 2' x 2' fluorescent fixtures. Any help in this matter is greatly appreciated.

Look for the amp rating on the ballast.

If necessary to convert into watts, actually called Volt-Amps (VA)

Multiply 120 volts, or the supply voltage you have, by amps to arrive at watts (VA)

Do not add the wattage listed on the lamps.

Are you saying the power consumption (wattage) is strictly a function of the balast, not the wattage of the tubes?

The total power consumption of a fluorescent fixture is a combination of the lamps AND ballast.

Like TK originally stated, the rating of the ballast is indicated right on it in amps. Multiply this by 120 and get the watts.

This number includes the lamps plus whatever the ballast itself draws (which is minute).

If the ballast is rate for several different lamps you DO have to take that into account.

Thank TK and stan, stan your explanation drove it home for me.

I am interpreting from a few posts at this site, two possible formulas:
1. Watts = (# tubes * tube watts * 1/ballast efficiency) + (ballast amp x supply volts)
2. Watts = input amp & supply volts

The problem is getting the specs to plug into either formula. I am trying to decide on purchase of shop lights. For example there are 4 electric ballast light fixtures at the big box that range in price from $12 to $40 for 2 tube x 4’ x T8 x 32 watts. None have shields; they do vary slightly in reflector. There are similar variations for 8’ units. There are no specs on the boxes or units other than the examples stated above. It is not possible to open the units to read the ballast specs (input amp, input watts, BEF, ballast factor, etc.). How does one determine the difference in power consumption, whether comparing T8 units to each other or versus T12 or T5, etc?